libceph: assume argonaut on the server side

No one is running pre-argonaut.  In addition one of the argonaut
features (NOSRCADDR) has been required since day one (and a half,
2.6.34 vs 2.6.35) of the kernel client.

Allow for the possibility of reusing these feature bits later.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>