scsi: target: Detect UNMAP support post configuration

On our backend we can do something similar to LIO where we can enable and
disable UNMAP support on the fly. In the SCSI/block layer we can detect
this by just doing a rescan. However, LIO cannot detect this change because
we only check during the initial configuration. This patch allows UNMAP
detection to also happen when the user tries to turn it on.

Link: https://lore.kernel.org/r/20220628200230.15052-6-michael.christie@oracle.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>