mds: flush after scrub repairs

mds: flush after scrub repairs

Otherwise, if we restart the MDS right after the scrub,
then scrub again, it will see the same inconsistency when
it looks at the on-disk state.

This involves adapting the use of ScrubHeader to be something
writeable during scrub to record whether repair happened.

Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit 7e52729699e87dd75022433153450fd4ba71b40e)