Btrfs: make sure btrfs_remove_free_space doesn't leak EAGAIN

btrfs_remove_free_space needs to make sure to set ret back to a
valid return value after setting it to EAGAIN, otherwise we return
it to the callers.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/free-space-cache.c

@@ -1868,6 +1868,7 @@ again:
 			ctl->total_bitmaps--;
 		}
 		kmem_cache_free(btrfs_free_space_cachep, info);
+		ret = 0;
 		goto out_lock;
 	}
 
@@ -1875,7 +1876,8 @@ again:
 		unlink_free_space(ctl, info);
 		info->offset += bytes;
 		info->bytes -= bytes;
-		link_free_space(ctl, info);
+		ret = link_free_space(ctl, info);
+		WARN_ON(ret);
 		goto out_lock;
 	}