ext4: fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of just letting the code
fall through to the next case.

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


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/03497331f088a938d7a728e7a689bd7953139429.1605896059.git.gustavoars@kernel.org


Signed-off-by: Theodore Ts'o <tytso@mit.edu>