bitmap: Update count after a merge

bitmap: Update count after a merge

We need an accurate count of the number of bits set in a bitmap
after a merge. In particular, since the merge operation short-circuits
a merge from an empty source, if you have bitmaps A, B, and C where
B started empty, then merge C into B, and B into A, an inaccurate
count meant that A did not get the contents of C.

In the worst case, we may falsely regard the bitmap as empty when
it has had new writes merged into it.

Fixes: be58721db
CC: [email protected]
Signed-off-by: Eric Blake <[email protected]>
Signed-off-by: John Snow <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-id: 20181002233314[email protected]
Signed-off-by: John Snow <[email protected]>