btrfs: make lock_extent_buffer_for_io() to be subpage compatible

For subpage metadata, we don't use page locking at all.  So just skip
the page locking part for subpage.  The rest of the function can be
reused.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>