rds_rdma_unuse() drops the mr reference count which it hasn't
taken. Correct way of removing mr is to remove mr from the tree
and then rdma_destroy_mr() it first, then rds_mr_put() to decrement
its reference count. Whichever thread holds last reference will free
the mr via rds_mr_put()

This bug was triggering weird null pointer crashes. One if the trace
for it is captured below.

BUG: unable to handle kernel NULL pointer dereference at
0000000000000104
IP: [<ffffffffa0899471>] rds_ib_free_mr+0x31/0x130 [rds_rdma]
PGD 4366fa067 PUD 4366f9067 PMD 0
Oops: 0000 [#1] SMP

[...]

task: ffff88046da6a000 ti: ffff88046da6c000 task.ti: ffff88046da6c000
RIP: 0010:[<ffffffffa0899471>]  [<ffffffffa0899471>]
rds_ib_free_mr+0x31/0x130 [rds_rdma]
RSP: 0018:ffff88046fa43bd8  EFLAGS: 00010286
RAX: 0000000071d38b80 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff880079e7ff40
RBP: ffff88046fa43bf8 R08: 0000000000000000 R09: 0000000000000000
R10: ffff88046fa43ca8 R11: ffff88046a802ed8 R12: ffff880079e7fa40
R13: 0000000000000000 R14: ffff880079e7ff40 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff88046fa40000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000104 CR3: 00000004366fb000 CR4: 00000000000006e0
Stack:
 ffff880079e7fa40 ffff880671d38f08 ffff880079e7ff40 0000000000000296
 ffff88046fa43c28 ffffffffa087a38b ffff880079e7fa40 ffff880671d38f10
 0000000000000000 0000000000000292 ffff88046fa43c48 ffffffffa087a3b6
Call Trace:
 <IRQ>
 [<ffffffffa087a38b>] rds_destroy_mr+0x8b/0xa0 [rds]
 [<ffffffffa087a3b6>] __rds_put_mr_final+0x16/0x30 [rds]
 [<ffffffffa087a492>] rds_rdma_unuse+0xc2/0x120 [rds]
 [<ffffffffa08766d3>] rds_recv_incoming_exthdrs+0x83/0xa0 [rds]
 [<ffffffffa0876782>] rds_recv_incoming+0x92/0x200 [rds]
 [<ffffffffa0895269>] rds_ib_process_recv+0x259/0x320 [rds_rdma]
 [<ffffffffa08962a8>] rds_ib_recv_tasklet_fn+0x1a8/0x490 [rds_rdma]
 [<ffffffff810dcd78>] ? __remove_hrtimer+0x58/0x90
 [<ffffffff810799e1>] tasklet_action+0xb1/0xc0
 [<ffffffff81079b52>] __do_softirq+0xe2/0x290
 [<ffffffff81079df6>] irq_exit+0xa6/0xb0
 [<ffffffff81613915>] do_IRQ+0x65/0xf0
 [<ffffffff816118ab>] common_interrupt+0x6b/0x6b

Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>