Under heavy iser target(scst) start/stop stress during login/logout
on iser intitiator side happened trace call provided below.

The function iscsi_iser_slave_alloc iser_conn pointer could be NULL,
due to the fact that function iscsi_iser_conn_stop can be called before
and free iser connection. Let's protect that flow by introducing global mutex.

BUG: unable to handle kernel paging request at 0000000000001018
IP: [<ffffffffc0426f7e>] iscsi_iser_slave_alloc+0x1e/0x50 [ib_iser]
Call Trace:
? scsi_alloc_sdev+0x242/0x300
scsi_probe_and_add_lun+0x9e1/0xea0
? kfree_const+0x21/0x30
? kobject_set_name_vargs+0x76/0x90
? __pm_runtime_resume+0x5b/0x70
__scsi_scan_target+0xf6/0x250
scsi_scan_target+0xea/0x100
iscsi_user_scan_session.part.13+0x101/0x130 [scsi_transport_iscsi]
? iscsi_user_scan_session.part.13+0x130/0x130 [scsi_transport_iscsi]
iscsi_user_scan_session+0x1e/0x30 [scsi_transport_iscsi]
device_for_each_child+0x50/0x90
iscsi_user_scan+0x44/0x60 [scsi_transport_iscsi]
store_scan+0xa8/0x100
? common_file_perm+0x5d/0x1c0
dev_attr_store+0x18/0x30
sysfs_kf_write+0x37/0x40
kernfs_fop_write+0x12c/0x1c0
__vfs_write+0x18/0x40
vfs_write+0xb5/0x1a0
SyS_write+0x55/0xc0

Fixes: 318d311e ("iser: Accept arbitrary sg lists mapping if the device supports it")
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Vladimir Neyelov <vladimirn@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimbeg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>