btrfs: assert tree is locked when clearing extent map from logging

When calling clear_em_logging() we should have a write lock on the extent
map tree, as we will try to merge the extent map with the previous and
next ones in the tree. So assert that we have a write lock.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -334,6 +334,8 @@ int unpin_extent_cache(struct extent_map_tree *tree, u64 start, u64 len,
 
 void clear_em_logging(struct extent_map_tree *tree, struct extent_map *em)
 {
+	lockdep_assert_held_write(&tree->lock);
+
 	clear_bit(EXTENT_FLAG_LOGGING, &em->flags);
 	if (extent_map_in_tree(em))
 		try_merge_map(tree, em);