ceph: fix wrong sizeof argument issue in register_session()

ceph: fix wrong sizeof argument issue in register_session()

The Coverity Scan service has detected the wrong sizeof
argument in register_session() [1]. The CID 1598909 defect
contains explanation: "The wrong sizeof value is used in
an expression or as argument to a function. The result is
an incorrect value that may cause unexpected program behaviors.
In register_session: The sizeof operator is invoked on
the wrong argument (CWE-569)".

The patch introduces a ptr_size variable that is initialized
by sizeof(struct ceph_mds_session *). And this variable is used
instead of sizeof(void *) in the code.

[1] https://scan5.scan.coverity.com/#/project-view/64304/10063?selectedIssue=1598909

Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>