scsi: scsi_transport_srp: Sanitize scsi_target_block/unblock sequences

The SCSI midlayer does not allow state transitions from SDEV_BLOCK to
SDEV_BLOCK so calling scsi_target_block() from __rport_fast_io_fail() is
wrong as the port is already blocked.  Similarly, we don't need to call
scsi_target_unblock() afterwards as the function has already done this.

Link: https://lore.kernel.org/r/20200728134833.42547-1-hare@suse.de
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>