Fix the "INVALID ARGUMENT" issue when we mount java client on sub dir

[taodd]

correct the way to choose a default cephfs data pool

Signed-off-by: dongdong tao [email protected]

[taodd]

I hava pulled a request in ceph project along with this one: https://github.com/ceph/ceph/pull/16671

[ukernel]

LGTM