scsi: core: Add scsi_{get,put}_internal_cmd() helpers

scsi: core: Add scsi_{get,put}_internal_cmd() helpers

Add helper functions to allow LLDDs to allocate and free internal commands.

[ bvanassche: changed the 'nowait' argument into a 'flags' argument. See also
  https://lore.kernel.org/linux-scsi/20211125151048.103910-3-hare@suse.de/ ]

Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://patch.msgid.link/20251031204029.2883185-7-bvanassche@acm.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>