scsi: target: tcm_loop: Use block cmd allocator for se_cmds

Make tcm_loop use the block layer cmd allocator for se_cmds instead of
using the tcm_loop_cmd_cache. In the future when we can use the host tags
for internal requests like TMFs we can completely kill the
tcm_loop_cmd_cache.

Link: https://lore.kernel.org/r/20210227170006.5077-19-michael.christie@oracle.com
Tested-by: Laurence Oberman <loberman@redhat.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>