ext4: annotate implicit fall throughs

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1). Fix them up.

This commit remove the following warnings:

  fs/ext4/hash.c:233:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
  fs/ext4/hash.c:246:15: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>