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>