The find_ref_head shouldn't return the first entry even if no exact match
is found. So move the hidden behavior to higher level.

Besides, remove the useless local variables in the btrfs_select_ref_head.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NLu Fengqi <lufq.fnst@cn.fujitsu.com>
[ reformat comment ]
Signed-off-by: NDavid Sterba <dsterba@suse.com>