This patch fixes the following KASAN complaint:

==================================================================
BUG: KASAN: stack-out-of-bounds in rxe_post_send+0x77d/0x9b0 [rdma_rxe]
Read of size 8 at addr ffff880061aef860 by task 01/1080

CPU: 2 PID: 1080 Comm: 01 Not tainted 4.16.0-rc3-dbg+ #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
Call Trace:
dump_stack+0x85/0xc7
print_address_description+0x65/0x270
kasan_report+0x231/0x350
rxe_post_send+0x77d/0x9b0 [rdma_rxe]
__ib_drain_sq+0x1ad/0x250 [ib_core]
ib_drain_qp+0x9/0x30 [ib_core]
srp_destroy_qp+0x51/0x70 [ib_srp]
srp_free_ch_ib+0xfc/0x380 [ib_srp]
srp_create_target+0x1071/0x19e0 [ib_srp]
kernfs_fop_write+0x180/0x210
__vfs_write+0xb1/0x2e0
vfs_write+0xf6/0x250
SyS_write+0x99/0x110
do_syscall_64+0xee/0x2b0
entry_SYSCALL_64_after_hwframe+0x42/0xb7

The buggy address belongs to the page:
page:ffffea000186bbc0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x4000000000000000()
raw: 4000000000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: 0000000000000000 ffffea000186bbe0 0000000000000000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff880061aef700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff880061aef780: 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00
>ffff880061aef800: f2 f2 f2 f2 f2 f2 f2 00 00 00 00 00 f2 f2 f2 f2
                                                      ^
ffff880061aef880: f2 f2 f2 00 00 00 00 00 00 00 00 00 00 00 f2 f2
ffff880061aef900: f2 f2 f2 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================

Fixes: 765d6774 ("IB: new common API for draining queues")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Steve Wise <swise@opengridcomputing.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: stable@vger.kernel.org
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>