mm: use pagevec_lookup_range_tag() in write_cache_pages()

Use pagevec_lookup_range_tag() in write_cache_pages() as it is
interested only in pages from given range.  Remove unnecessary code
resulting from this.

Link: http://lkml.kernel.org/r/20171009151359.31984-12-jack@suse.cz


Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>