afs: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding multiple fallthrough pseudo-keywords in
places where the code is intended to fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-hardening@vger.kernel.org
Link: https://lore.kernel.org/r/51150b54e0b0431a2c401cd54f2c4e7f50e94601.1605896059.git.gustavoars@kernel.org/ # v1
Link: https://lore.kernel.org/r/20210420211615.GA51432@embeddedor/

 # v2
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>