RDMA/rxe: Fix an out-of-bounds read

RDMA/rxe: Fix an out-of-bounds read

This patch avoids that KASAN reports the following when the SRP initiator
calls srp_post_send():

==================================================================
BUG: KASAN: stack-out-of-bounds in rxe_post_send+0x5c4/0x980 [rdma_rxe]
Read of size 8 at addr ffff880066606e30 by task 02-mq/1074

CPU: 2 PID: 1074 Comm: 02-mq Not tainted 4.16.0-rc3-dbg+ #1
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+0x5c4/0x980 [rdma_rxe]
srp_post_send.isra.16+0x149/0x190 [ib_srp]
srp_queuecommand+0x94d/0x1670 [ib_srp]
scsi_dispatch_cmd+0x1c2/0x550 [scsi_mod]
scsi_queue_rq+0x843/0xa70 [scsi_mod]
blk_mq_dispatch_rq_list+0x143/0xac0
blk_mq_do_dispatch_ctx+0x1c5/0x260
blk_mq_sched_dispatch_requests+0x2bf/0x2f0
__blk_mq_run_hw_queue+0xdb/0x160
__blk_mq_delay_run_hw_queue+0xba/0x100
blk_mq_run_hw_queue+0xf2/0x190
blk_mq_sched_insert_request+0x163/0x2f0
blk_execute_rq+0xb0/0x130
scsi_execute+0x14e/0x260 [scsi_mod]
scsi_probe_and_add_lun+0x366/0x13d0 [scsi_mod]
__scsi_scan_target+0x18a/0x810 [scsi_mod]
scsi_scan_target+0x11e/0x130 [scsi_mod]
srp_create_target+0x1522/0x19e0 [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:ffffea0001998180 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x4000000000000000()
raw: 4000000000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: dead000000000100 dead000000000200 0000000000000000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff880066606d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1
ffff880066606d80: f1 00 f2 f2 f2 f2 f2 f2 f2 00 00 f2 f2 f2 f2 f2
>ffff880066606e00: f2 00 00 00 00 00 f2 f2 f2 f3 f3 f3 f3 00 00 00
                                    ^
ffff880066606e80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff880066606f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================

Fixes: 8700e3e7c485 ("Soft RoCE driver")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Moni Shoua <monis@mellanox.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>