This saves a few calls to compound_head(), including one in
filemap_update_page().  Shrinks the kernel by 78 bytes.
Signed-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NWilliam Kucharski <william.kucharski@oracle.com>