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modern XFS this option is obsolete, but for older kernels it was not the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>
Merge pull request #2497 from ceph/wip-xfs-inode64

ceph-disk: mount xfs with inode64 by default

Reviewed-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>