IB/rxe: Fix kernel panic from skb destructor

IB/rxe: Fix kernel panic from skb destructor

In the time between rxe_send has finished and skb destructor
called, the QP's ref count might be 0, leading to a possible
QP destruction. This will lead to a kernel panic when the destructor
dereferences the QP.

The operation of incrementing QP ref count at rxe_send and decrementing
from skb destructor will prevent this crash.

BUG: unable to handle kernel NULL pointer dereference at 000000000000072c
IP: [<ffffffffa05df765>] rxe_skb_tx_dtor+0x15/0x50 [rdma_rxe]
PGD 0 [16240.211178]
Oops: 0002 [#1] SMP
CPU: 3 PID: 0 Comm: swapper/3 Tainted: G           OE   4.9.0-mlnx #1
Hardware name: Red Hat KVM, BIOS Bochs 01/01/2011
task: ffff88042d6b1480 task.stack: ffffc90001904000
RIP: 0010:[<ffffffffa05df765>]  [<ffffffffa05df765>] rxe_skb_tx_dtor+0x15/0x50 [rdma_rxe]
RSP: 0018:ffff88043fcc3df0  EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff880429684700 RCX: ffff88042d248200
RDX: 00000000ffffffff RSI: 00000000fffffe01 RDI: ffff880429684700
RBP: ffff88043fcc3e00 R08: ffff88043fcda240 R09: 00000000ff2d1de6
R10: 0000000000000000 R11: 00000000f49cf6fe R12: ffff880429684700
R13: ffffffff81893f96 R14: ffffffff817d66f0 R15: ffff880427f74200
FS:  0000000000000000(0000) GS:ffff88043fcc0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000000072c CR3: 000000041d3df000 CR4: 00000000000006e0
Stack:
 ffffffff817b29cf ffff880429684700 ffff88043fcc3e18 ffffffff817b42c2
 ffff880429684700 ffff88043fcc3e40 ffffffff817b4332 ffff880429684700
 ffff880427f74238 ffff880427f74228 ffff88043fcc3e58 ffffffff81893f96
Call Trace:
 <IRQ> [16240.336345]  [<ffffffff817b29cf>] ? skb_release_head_state+0x4f/0xb0
 [<ffffffff817b42c2>] skb_release_all+0x12/0x30
 [<ffffffff817b4332>] kfree_skb+0x32/0x90
 [<ffffffff81893f96>] ndisc_error_report+0x36/0x40
 [<ffffffff817d4de1>] neigh_invalidate+0x81/0xf0
 [<ffffffff817d68f7>] neigh_timer_handler+0x207/0x2b0
 [<ffffffff81109295>] call_timer_fn+0x35/0x120
 [<ffffffff81109db7>] run_timer_softirq+0x1d7/0x460
 [<ffffffff8106155e>] ? kvm_sched_clock_read+0x1e/0x30
 [<ffffffff810366b9>] ? sched_clock+0x9/0x10
 [<ffffffff810cfed2>] ? sched_clock_cpu+0x72/0xa0
 [<ffffffff818dd537>] __do_softirq+0xd7/0x289
 [<ffffffff810a6c95>] irq_exit+0xb5/0xc0
 [<ffffffff818dd372>] smp_apic_timer_interrupt+0x42/0x50
 [<ffffffff818dc682>] apic_timer_interrupt+0x82/0x90
 <EOI> [16240.395776]  [<ffffffff818da156>] ? native_safe_halt+0x6/0x10
 [<ffffffff818d9e6e>] default_idle+0x1e/0xd0
 [<ffffffff8103797f>] arch_cpu_idle+0xf/0x20
 [<ffffffff818da2c5>] default_idle_call+0x35/0x40
 [<ffffffff810e3eb5>] cpu_startup_entry+0x185/0x210
 [<ffffffff81050433>] start_secondary+0x103/0x130
RIP  [<ffffffffa05df765>] rxe_skb_tx_dtor+0x15/0x50 [rdma_rxe]

Fixes: c4397c211d7d ("Soft RoCE driver")
Signed-off-by: Yonatan Cohen <yonatanc@mellanox.com>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>