scsi: ibmvscsi: Protect ibmvscsi_head from concurrent modificaiton

For each ibmvscsi host created during a probe or destroyed during a remove
we either add or remove that host to/from the global ibmvscsi_head
list. This runs the risk of concurrent modification.

This patch adds a simple spinlock around the list modification calls to
prevent concurrent updates as is done similarly in the ibmvfc driver and
ipr driver.

Fixes: 32d6e4b6 ("scsi: ibmvscsi: add vscsi hosts to global list_head")
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>