scsi: arcmsr: Buffer overflow in arcmsr_iop_message_xfer()

We need to put an upper bound on "user_len" so the memcpy() doesn't
overflow.

Cc: <stable@vger.kernel.org>
Reported-by: Marco Grassi <marco.gra@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>