bcache: fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/md/bcache/btree.c:2220:5: warning:
 symbol 'btree_insert_fn' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/btree.c

@@ -2217,7 +2217,7 @@ struct btree_insert_op {
 	struct bkey	*replace_key;
 };
 
-int btree_insert_fn(struct btree_op *b_op, struct btree *b)
+static int btree_insert_fn(struct btree_op *b_op, struct btree *b)
 {
 	struct btree_insert_op *op = container_of(b_op,
 					struct btree_insert_op, op);