add comment for ext4_dir_entry_2 file_type member

Signed-off-by: Jonathan Grant <jg@jguk.org>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/ad3290d5-86af-99c1-f9d5-cd1bab710429@jguk.org


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