btrfs: integrating raid-repair and scrub-fixup-nodatasum

btrfs: integrating raid-repair and scrub-fixup-nodatasum

This ties nodatasum fixup in scrub together with raid repair patches. While
both series are working fine alone, scrub will report uncorrectable errors
if they occur in a nodatasum extent *and* the page is in the page cache.

Previously, we would have triggered readpage to find good data and do the
repair. However, readpage wouldn't read anything in the case where the page
is up to date in the cache. So, we simply take that good data we have and
call repair_io_failure directly (unless the page in the cache is dirty).

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
(cherry picked from commit 5da6fcbc4eb50c0f55d520750332f5a6ab13508c)