ext4: optimize __ext4_check_dir_entry()

Make __ext4_check_dir_entry() a bit easier to understand, and reduce
the object size of the function by over 11%.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20191209004346.38526-1-tytso@mit.edu


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