__get_first_dirty_index() wants to lookup only the first dirty page
after given index.  There's no point in using pagevec_lookup_tag() for
that.  Just use find_get_pages_tag() directly.

Link: http://lkml.kernel.org/r/20171009151359.31984-8-jack@suse.czSigned-off-by: NJan Kara <jack@suse.cz>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Reviewed-by: NDaniel Jordan <daniel.m.jordan@oracle.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>