osd: do not leak Session* ref in _send_boot()

The get_priv() call returns a ref; make sure we drop it if it exists.
This doesn't happen on every run because usually it is NULL and we take
the other path; it's only after the OSD has been marked down that we reach
the second path.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2190 from ceph/wip-osd-leaks

osd: do not leak Session* ref in _send_boot()

Reviewed-by: Greg Farnum <greg@inktank.com>