scsi: mpt3sas: Fix _transport_smp_handler() error path

This patch avoids that smatch complains about a double unlock on
ioc->transport_cmds.mutex.

Fixes: 651a0136 ("scsi: scsi_transport_sas: switch to bsg-lib for SMP passthrough")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sathya Prakash <sathya.prakash@broadcom.com>
Cc: Chaitra P B <chaitra.basappa@broadcom.com>
Cc: Suganath Prabu Subramani <suganath-prabu.subramani@broadcom.com>
Cc: stable@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>