ceph-disk: mount xfs with inode64 by default

We did this forever ago with mkcephfs, but ceph-disk didn't.  Note that for
modern XFS this option is obsolete, but for older kernels it was not the
default.

Backport: firefly
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 11496399ef318498c11e551f139d96db52d3309c)