scsi: target: core: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a couple of
warnings by explicitly adding a break statement and a fallthrough
pseudo-keyword instead of letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/r/761d6f755e8a6f8a6daebd1e5c1394167e5c780a.1605896059.git.gustavoars@kernel.org


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>