ceph.spec.in: use multi-threaded xz compression

ceph.spec.in: use multi-threaded xz compression

so we can build the compressed src rpm and binary rpms with smaller
latency.

see also
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/7.3_release_notes/technology_previews_installation_and_booting

by default, rpm uses

w9.gzdio

which means: gzip level 9. but since xz is able to compress using
multiple threads, let's switch to xz.

xz's default compression level is 6, but ceph's source and binary
packages are relatively large. let's use a higher compression level.

Signed-off-by: Kefu Chai <kchai@redhat.com>