btrfs: avoid searching twice for previous node when merging free space entries

At try_merge_free_space(), avoid calling twice rb_prev() to find the
previous node, as that requires looping through the red black tree, so
store the result of the rb_prev() call and then use it.
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/free-space-cache.c

@@ -2449,6 +2449,7 @@ static bool try_merge_free_space(struct btrfs_free_space_ctl *ctl,
 	u64 offset = info->offset;
 	u64 bytes = info->bytes;
 	const bool is_trimmed = btrfs_free_space_trimmed(info);
+	struct rb_node *right_prev = NULL;
 
 	/*
 	 * first we want to see if there is free space adjacent to the range we
@@ -2456,9 +2457,11 @@ static bool try_merge_free_space(struct btrfs_free_space_ctl *ctl,
 	 * cover the entire range
 	 */
 	right_info = tree_search_offset(ctl, offset + bytes, 0, 0);
-	if (right_info && rb_prev(&right_info->offset_index))
-		left_info = rb_entry(rb_prev(&right_info->offset_index),
-				     struct btrfs_free_space, offset_index);
+	if (right_info)
+		right_prev = rb_prev(&right_info->offset_index);
+
+	if (right_prev)
+		left_info = rb_entry(right_prev, struct btrfs_free_space, offset_index);
 	else if (!right_info)
 		left_info = tree_search_offset(ctl, offset - 1, 0, 0);