afs: yfsclient: Mark expected switch fall-throughs

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

This patch fixes the following warnings:

fs/afs/yfsclient.c: In function ‘yfs_deliver_fs_fetch_opaque_acl’:
fs/afs/yfsclient.c:1984:19: warning: this statement may fall through [-Wimplicit-fallthrough=]
   call->unmarshall++;
   ~~~~~~~~~~~~~~~~^~
fs/afs/yfsclient.c:1987:2: note: here
  case 1:
  ^~~~
fs/afs/yfsclient.c:2005:19: warning: this statement may fall through [-Wimplicit-fallthrough=]
   call->unmarshall++;
   ~~~~~~~~~~~~~~~~^~
fs/afs/yfsclient.c:2008:2: note: here
  case 2:
  ^~~~
fs/afs/yfsclient.c:2014:19: warning: this statement may fall through [-Wimplicit-fallthrough=]
   call->unmarshall++;
   ~~~~~~~~~~~~~~~~^~
fs/afs/yfsclient.c:2017:2: note: here
  case 3:
  ^~~~
fs/afs/yfsclient.c:2035:19: warning: this statement may fall through [-Wimplicit-fallthrough=]
   call->unmarshall++;
   ~~~~~~~~~~~~~~~~^~
fs/afs/yfsclient.c:2038:2: note: here
  case 4:
  ^~~~
fs/afs/yfsclient.c:2047:19: warning: this statement may fall through [-Wimplicit-fallthrough=]
   call->unmarshall++;
   ~~~~~~~~~~~~~~~~^~
fs/afs/yfsclient.c:2050:2: note: here
  case 5:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Also, fix some commenting style issues.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>