ib_srpt: Fix a use-after-free in srpt_close_ch()

Avoid that KASAN reports the following:

BUG: KASAN: use-after-free in srpt_close_ch+0x4f/0x1b0 [ib_srpt]
Read of size 4 at addr ffff880151180cb8 by task check/4681

CPU: 15 PID: 4681 Comm: check Not tainted 4.18.0-rc2-dbg+ #4
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
Call Trace:
 dump_stack+0xa4/0xf5
 print_address_description+0x6f/0x270
 kasan_report+0x241/0x360
 __asan_load4+0x78/0x80
 srpt_close_ch+0x4f/0x1b0 [ib_srpt]
 srpt_set_enabled+0xf7/0x1e0 [ib_srpt]
 srpt_tpg_enable_store+0xb8/0x120 [ib_srpt]
 configfs_write_file+0x14e/0x1d0 [configfs]
 __vfs_write+0xd2/0x3b0
 vfs_write+0x101/0x270
 ksys_write+0xab/0x120
 __x64_sys_write+0x43/0x50
 do_syscall_64+0x77/0x230
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: aaf45bd8 ("IB/srpt: Detect session shutdown reliably")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/srpt/ib_srpt.c

@@ -1834,8 +1834,7 @@ static bool srpt_close_ch(struct srpt_rdma_ch *ch)
 	int ret;
 
 	if (!srpt_set_ch_state(ch, CH_DRAINING)) {
-		pr_debug("%s-%d: already closed\n", ch->sess_name,
-			 ch->qp->qp_num);
+		pr_debug("%s: already closed\n", ch->sess_name);
 		return false;
 	}