os/bluestore: defer csum calcuations sometimes

os/bluestore: defer csum calcuations sometimes

When we are doing a partial chunk overwrite, we need to defer the csum_data
update.  Otherwise, another write in the same transaction might need to
read part of the chunk, not find the data in the buffer cache, read it
from disk, and fail the csum check.

This patch defers the calculation until after we've build the transaction
and are about to commit to the kv store.

Signed-off-by: Sage Weil <sage@redhat.com>