bcachefs: clear path->should_be_locked in bch2_btree_key_cache_drop()

bch2_btree_key_cache_drop() evicts the key cache entry - it's used when
we're doing an update that bypasses the key cache, because for cache
coherency reasons a key can't be in the key cache unless it also exists
in the btree - i.e. creates have to bypass the cache.

After evicting, the path no longer points to a key cache key, and
relock() will always fail if should_be_locked is true.

Prep for improving path->should_be_locked assertions
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_key_cache.c

@@ -726,6 +726,7 @@ void bch2_btree_key_cache_drop(struct btree_trans *trans,
 
 	mark_btree_node_locked(trans, path, 0, BTREE_NODE_UNLOCKED);
 	btree_path_set_dirty(path, BTREE_ITER_NEED_TRAVERSE);
+	path->should_be_locked = false;
 }
 
 static unsigned long bch2_btree_key_cache_scan(struct shrinker *shrink,