scsi: target: pscsi: Clean up after failure in pscsi_map_sg()

If pscsi_map_sg() fails, make sure to drop references to already allocated
bios.

Link: https://lore.kernel.org/r/20210323212431.15306-2-mwilck@suse.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>