fs: don't call ->writepage from __mpage_writepage

All callers of mpage_writepage use block_write_full_page as their
->writepage implementation when called from mpage_writepages
(although for ntfs3 this is obsfucated a bit).

Just call block_write_full_page directly instead of going through
the ->writepage indirection.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>