f2fs: allow direct io of pinned files for zoned storage

Since the allocation happens in conventional LU for zoned storage, we
can allow direct io for that.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>