bcachefs: Fix locking in allocator thread

gc lock must be held while invalidating buckets - fixes
"1f7a95698e bcachefs: Invalidate buckets when writing to alloc btree"
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc.c

@@ -964,16 +964,21 @@ static int bch2_allocator_thread(void *arg)
 		if (ret)
 			goto stop;
 
+		down_read(&c->gc_lock);
+
 		ret = bch2_invalidate_buckets(c, ca);
-		if (ret)
+		if (ret) {
+			up_read(&c->gc_lock);
 			goto stop;
+		}
 
-		if (!fifo_empty(&ca->free_inc))
+		if (!fifo_empty(&ca->free_inc)) {
+			up_read(&c->gc_lock);
 			continue;
+		}
 
 		pr_debug("free_inc now empty");
 
-		down_read(&c->gc_lock);
 		do {
 			if (test_bit(BCH_FS_GC_FAILURE, &c->flags)) {
 				up_read(&c->gc_lock);