os/bluestore: only direct write into unused blob space

os/bluestore: only direct write into unused blob space

We can only do a direct write into an already-allocated blob once, if that
range hasn't yet been used.  Once it has been used, it is much to complex
to keep track of when all references to it have committed to disk before
reusing it, so we don't try to handle that case at all.

Since the range has never been used, we can assert that there are no
references to it.

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