btrfs: add missing annotation for btrfs_lock_cluster()

Sparse reports a warning at btrfs_lock_cluster()

warning: context imbalance in btrfs_lock_cluster()
	- wrong count

The root cause is the missing annotation at btrfs_lock_cluster()
Add the missing __acquires(&cluster->refill_lock) annotation.

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>