mgr: fix PyObject leak in the remote() pickle calls

dispatch_remote() and ceph_dispatch_remote() invoke pickle.loads/dumps via
PyObject_CallMethodObjArgs(pmodule, PyUnicode_FromString("loads"), ...). The
method-name string created by PyUnicode_FromString is a new reference that
CallMethodObjArgs does not steal, and it was never released, so every cross
-module remote() call leaked three PyUnicode objects on the active mgr.

Use PyObject_CallMethod(pmodule, "loads", "(O)", arg), which takes the method
name as a C string and manages it internally, so there is nothing to leak.
The "(O)" format wraps the argument in a one-tuple so it reaches the callee as
a single object. A bare "O" passes the argument through directly, and when it
is a tuple (as remote()'s *args always is, including the empty tuple from a
no-argument call) PyObject_CallMethod unpacks it and calls dumps() with the
wrong arguments, e.g. dumps() with none.

Fixes: https://tracker.ceph.com/issues/77724
Signed-off-by: Kefu Chai <k.chai@proxmox.com>