ext4: Don't use ext4_dec_count() if not needed

The ext4_dec_count() function is only needed when dropping the i_nlink
count on inodes which are (or which could be) directories.  If we
*know* that the inode in question can't possibly be a directory, use
drop_nlink or clear_nlink() if we know i_nlink is 1.

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