bcachefs: Fix another lockdep splat

vfree() can allocate memory, so we need to call memalloc_nofs_save().
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_cache.c

@@ -253,7 +253,7 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 	unsigned long can_free;
 	unsigned long touched = 0;
 	unsigned long freed = 0;
-	unsigned i;
+	unsigned i, flags;
 
 	if (btree_shrinker_disabled(c))
 		return SHRINK_STOP;
@@ -264,6 +264,8 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 	else if (!mutex_trylock(&bc->lock))
 		return -1;
 
+	flags = memalloc_nofs_save();
+
 	/*
 	 * It's _really_ critical that we don't free too many btree nodes - we
 	 * have to always leave ourselves a reserve. The reserve is how we
@@ -327,6 +329,7 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 			clear_btree_node_accessed(b);
 	}
 
+	memalloc_nofs_restore(flags);
 	mutex_unlock(&bc->lock);
 out:
 	return (unsigned long) freed * btree_pages(c);