Lockdep complains about potential recursive locking during mount
because the client configuration log is holding a lock on the MGC
obd_device to prevent it from being torn down, while also getting
mutexes on the MDC and OSC devices as they are instantiated:

 Lustre: Mounted myth-client
=============================================
[ INFO: possible recursive locking detected ]
4.7.0-rc2-vm-nfs+ #127 Tainted: G         C
---------------------------------------------

 May be due to missing lock nesting notation
2 locks held by ll_cfg_requeue/5928:
 #0:  (&cli->cl_sem){.+.+.+}, at: mgc_requeue_thread+0x15d/0x730 [mgc]
 #1:  (&cld->cld_lock){+.+.+.}, at: mgc_process_log+0x5e/0xf80 [mgc]
CPU: 0 PID: 5928 Comm: ll_cfg_requeue
Call Trace:
 [<ffffffff814a0855>] dump_stack+0x86/0xc1
 [<ffffffff810e7766>] __lock_acquire+0x726/0x1210
 [<ffffffff810e86be>] lock_acquire+0xfe/0x1f0
 [<ffffffff81888171>] down_read+0x51/0xa0
 [<ffffffffa04a8477>] sptlrpc_conf_client_adapt+0x47/0x150 [ptlrpc]
 [<ffffffffa0186b16>] mdc_set_info_async+0x2b6/0x470 [mdc]
 [<ffffffffa0294090>] class_notify_sptlrpc_conf+0x190/0x360 [obdclass]
 [<ffffffffa01a9e85>] mgc_process_log+0x925/0xf80 [mgc]
 [<ffffffffa01abafa>] mgc_requeue_thread+0x1fa/0x730 [mgc]
 [<ffffffff810af331>] kthread+0x101/0x120
 [<ffffffff8188ad6f>] ret_from_fork+0x1f/0x40

Add a separate lock class for the MGC callpath, since it will always
be held first, and none of the other obd_device locks should ever
be held concurrently.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>