os/bluestore: write padded data into buffer cache

os/bluestore: write padded data into buffer cache

We rely on the buffer cache to avoid reading any deferred write data. In
order for that to work, we have to ensure the entire block whose
overwrite is deferred is in the buffer cache.  Otherwise, a write to 0~5
that results in a deferred write could break a subsequent read from 5~5
that reads the same block from disk before the deferred write lands.

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