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>

fs/btrfs/extent_io.c

@@ -3967,7 +3967,13 @@ static noinline_for_stack int lock_extent_buffer_for_io(struct extent_buffer *eb
 
 	btrfs_tree_unlock(eb);
 
-	if (!ret)
+	/*
+	 * Either we don't need to submit any tree block, or we're submitting
+	 * subpage eb.
+	 * Subpage metadata doesn't use page locking at all, so we can skip
+	 * the page locking.
+	 */
+	if (!ret || fs_info->sectorsize < PAGE_SIZE)
 		return ret;
 
 	num_pages = num_extent_pages(eb);