os/bluestore: return blocks allocated from allocate()

os/bluestore: return blocks allocated from allocate()

Instead of having a separate output argument with the number of
blocks allocated, just return it via the return value.  Simplifies
the calling convention.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 95c78706317f2fe61b8f48de5e8539ed49221d64)