dm bitset: only flush the current word if it has been dirtied

This change offers a big performance boost for dm-era.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --cc drivers/md/persistent-data/dm-bitset.c
Simple merge

diff --cc drivers/md/persistent-data/dm-bitset.h
Simple merge