fs: remove the NULL get_block case in mpage_writepages

No one calls mpage_writepages with a NULL get_block paramter, so remove
support for that case.

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