ext4: Fix BUG_ON at fs/buffer.c:652 in no journal mode

Calls to ext4_handle_dirty_metadata should only pass in an inode
pointer for inode-specific metadata, and not for shared metadata
blocks such as inode table blocks, block group descriptors, the
superblock, etc.

The BUG_ON can get tripped when updating a special device (such as a
block device) that is opened (so that i_mapping is set in
fs/block_dev.c) and the file system is mounted in no journal mode.

Addresses-Google-Bug: #2404870

Signed-off-by: Curt Wohlgemuth <curtw@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ialloc.c

@@ -898,7 +898,7 @@ repeat_in_this_group:
 				BUFFER_TRACE(inode_bitmap_bh,
 					"call ext4_handle_dirty_metadata");
 				err = ext4_handle_dirty_metadata(handle,
-								 inode,
+								 NULL,
 							inode_bitmap_bh);
 				if (err)
 					goto fail;