filestore: fix config observer
filestore: fix config observer

Actually, I don't think this was fully implemented to begin with, so it's
not a 'fix' per se.  This will let you use injectargs to adjust the
filestore config options during runtime.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph