ext4: allow unlocked direct IO when pages are cached

Currently we do not allow unlocked (meaning without inode_lock) direct
IO when the file has any pages cached. This check is not needed anymore
as we keep inode lock until ext4_direct_IO_write() and thus can happily
writeback and evict any pages conflicting with current direct IO write.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>