btrfs: don't stop integrity writeback too early

btrfs: don't stop integrity writeback too early

extent_write_cache_pages stops writing pages as soon as nr_to_write hits
zero.  That is the right thing for opportunistic writeback, but incorrect
for data integrity writeback, which needs to ensure that no dirty pages
are left in the range.  Thus only stop the writeback for WB_SYNC_NONE
if nr_to_write hits 0.

This is a port of write_cache_pages changes in commit 05fe478dd04e
("mm: write_cache_pages integrity fix").

Note that I've only trigger the problem with other changes to the btrfs
writeback code, but this condition seems worthwhile fixing anyway.

CC: stable@vger.kernel.org # 4.14+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
[ updated comment ]
Signed-off-by: David Sterba <dsterba@suse.com>