ceph: pass unhandled ceph.* setxattrs through to MDS

If we do not specifically understand a setxattr on a ceph.* virtual
xattr, send it through to the MDS.  This allows us to implement new
functionality via the MDS without direct support on the client side.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>