afs: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that in many cases I placed a /* Fall through */ comment
at the bottom of the case, which what GCC is expecting to find.

In other cases I had to tweak a bit the format of the comments.

This patch suppresses ALL missing-break-in-switch false positives
in fs/afs

Addresses-Coverity-ID: 115042 ("Missing break in switch")
Addresses-Coverity-ID: 115043 ("Missing break in switch")
Addresses-Coverity-ID: 115045 ("Missing break in switch")
Addresses-Coverity-ID: 1357430 ("Missing break in switch")
Addresses-Coverity-ID: 115047 ("Missing break in switch")
Addresses-Coverity-ID: 115050 ("Missing break in switch")
Addresses-Coverity-ID: 115051 ("Missing break in switch")
Addresses-Coverity-ID: 1467806 ("Missing break in switch")
Addresses-Coverity-ID: 1467807 ("Missing break in switch")
Addresses-Coverity-ID: 1467811 ("Missing break in switch")
Addresses-Coverity-ID: 115041 ("Missing break in switch")
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>