ceph: pass cred pointer to ceph_mds_auth_match()

In the Linux kernel, the following vulnerability has been resolved: ceph: pass cred pointer to ceph_mds_auth_match() This eliminates a redundant get_current_cred() call, because ceph_mds_check_access() has already obtained this pointer. As a side effect, this also fixes a reference leak in ceph_mds_auth_match(): by omitting the get_current_cred() call, no additional cred reference is taken.