Writing to ramfs files with O_SYNC dereferences null in mpage_writepages(),
due to null a_ops->writepage.  The call path is

	generic_file_aio_write_nolock
	->generic_osync_inode
	  ->write_inode_now
	    ->__writeback_single_inode
	      ->__sync_single_inode
	        ->do_writepages
	          ->mpage_writepages

The best fix for this is to visit all ram-backed filesystems and give them a
no-op a_ops.writepages.  But baling out if the file is memory-backed is a
sufficient coverall and is how we handle this in __filemap_fdatawrite().
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>