Two parallel calls to batadv_neigh_node_new() might race for creating
and adding the same neig_node. Fix this by including the check for any
already existing, identical neigh_node within the spin-lock.

This fixes splats like the following:

[  739.535069] ------------[ cut here ]------------
[  739.535079] WARNING: CPU: 0 PID: 0 at /usr/src/batman-adv/git/batman-adv/net/batman-adv/bat_iv_ogm.c:1004 batadv_iv_ogm_process_per_outif+0xe3f/0xe60 [batman_adv]()
[  739.535092] too many matching neigh_nodes
[  739.535094] Modules linked in: dm_mod tun ip6table_filter ip6table_mangle ip6table_nat nf_nat_ipv6 ip6_tables xt_nat iptable_nat nf_nat_ipv4 nf_nat xt_TCPMSS xt_mark iptable_mangle xt_tcpudp xt_conntrack iptable_filter ip_tables x_tables ip_gre ip_tunnel gre bridge stp llc thermal_sys kvm_intel kvm crct10dif_pclmul crc32_pclmul sha256_ssse3 sha256_generic hmac drbg ansi_cprng aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd evdev pcspkr ip6_gre ip6_tunnel tunnel6 batman_adv(O) libcrc32c nf_conntrack_ipv6 nf_defrag_ipv6 nf_conntrack_ipv4 nf_defrag_ipv4 nf_conntrack autofs4 ext4 crc16 mbcache jbd2 xen_netfront xen_blkfront crc32c_intel
[  739.535177] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W  O    4.2.0-0.bpo.1-amd64 #1 Debian 4.2.6-3~bpo8+2
[  739.535186]  0000000000000000 ffffffffa013b050 ffffffff81554521 ffff88007d003c18
[  739.535201]  ffffffff8106fa01 0000000000000000 ffff8800047a087a ffff880079c3a000
[  739.735602]  ffff88007b82bf40 ffff88007bc2d1c0 ffffffff8106fa7a ffffffffa013aa8e
[  739.735624] Call Trace:
[  739.735639]  <IRQ>  [<ffffffff81554521>] ? dump_stack+0x40/0x50
[  739.735677]  [<ffffffff8106fa01>] ? warn_slowpath_common+0x81/0xb0
[  739.735692]  [<ffffffff8106fa7a>] ? warn_slowpath_fmt+0x4a/0x50
[  739.735715]  [<ffffffffa012448f>] ? batadv_iv_ogm_process_per_outif+0xe3f/0xe60 [batman_adv]
[  739.735740]  [<ffffffffa0124813>] ? batadv_iv_ogm_receive+0x363/0x380 [batman_adv]
[  739.735762]  [<ffffffffa0124813>] ? batadv_iv_ogm_receive+0x363/0x380 [batman_adv]
[  739.735783]  [<ffffffff810b0841>] ? __raw_callee_save___pv_queued_spin_unlock+0x11/0x20
[  739.735804]  [<ffffffffa012cb39>] ? batadv_batman_skb_recv+0xc9/0x110 [batman_adv]
[  739.735825]  [<ffffffff81464891>] ? __netif_receive_skb_core+0x841/0x9a0
[  739.735838]  [<ffffffff810b0841>] ? __raw_callee_save___pv_queued_spin_unlock+0x11/0x20
[  739.735853]  [<ffffffff81465681>] ? process_backlog+0xa1/0x140
[  739.735864]  [<ffffffff81464f1a>] ? net_rx_action+0x20a/0x320
[  739.735878]  [<ffffffff81073aa7>] ? __do_softirq+0x107/0x270
[  739.735891]  [<ffffffff81073d82>] ? irq_exit+0x92/0xa0
[  739.735905]  [<ffffffff8137e0d1>] ? xen_evtchn_do_upcall+0x31/0x40
[  739.735924]  [<ffffffff8155b8fe>] ? xen_do_hypervisor_callback+0x1e/0x40
[  739.735939]  <EOI>  [<ffffffff810013aa>] ? xen_hypercall_sched_op+0xa/0x20
[  739.735965]  [<ffffffff810013aa>] ? xen_hypercall_sched_op+0xa/0x20
[  739.735979]  [<ffffffff8100a39c>] ? xen_safe_halt+0xc/0x20
[  739.735991]  [<ffffffff8101da6c>] ? default_idle+0x1c/0xa0
[  739.736004]  [<ffffffff810abf6b>] ? cpu_startup_entry+0x2eb/0x350
[  739.736019]  [<ffffffff81b2af5e>] ? start_kernel+0x480/0x48b
[  739.736032]  [<ffffffff81b2d116>] ? xen_start_kernel+0x507/0x511
[  739.736048] ---[ end trace c106bb901244bc8c ]---

Fixes: f987ed6e

 ("batman-adv: protect neighbor list with rcu locks")
Reported-by: Martin Weinelt <martin@darmstadt.freifunk.net>
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>