ext4: correct comment references to ext4_ext_direct_IO()

Commit 914f82a3 "ext4: refactor direct IO code" deleted
ext4_ext_direct_IO(), but references to that function remain in
comments.  Update them to refer to ext4_direct_IO_write().

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>