ext4: change to use fallthrough macro

Change to use fallthrough macro in switch case.

Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200810114435.24182-1-luoshijie1@huawei.com


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