cmake: use ceph repo with tag

cmake: use ceph repo with tag

to address following warning:

fatal: reference is not a tree: 7f02f21f53ccd5e2448086f8e9015489693dd2dc
CMake Error at /home/jenkins-build/build/workspace/ceph-pull-requests/build/fio_ext-prefix/tmp/fio_ext-gitclone.cmake:40 (message):
  Failed to checkout tag: '7f02f21f53ccd5e2448086f8e9015489693dd2dc'

it seems that the shallow option does not work with a sha1 tag option,
let's continue using the ceph repo with a tag.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit d7771002ef9a455390aadbcc84287dd9b738c629)