fix unit of rs_same_csums accounting

Depending on resync request size,
we need to account for more than one bit.

Impact: cosmetic

If SyncTarget reported correctly 100% equal checksums,
the SyncSource usually reported 12% equal checksums instead,
because it only counted requests, we typically do 32k resync requests,
and the bitmap granularity is still 4k.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>