MClientRequest: handle ext_num_retry and ext_num_fwd from ceph_mds_request_head_legacy

MClientRequest: handle ext_num_retry and ext_num_fwd from ceph_mds_request_head_legacy

When a client is too old and uses struct ceph_mds_request_head_legacy we must
fill new ext_num_retry and ext_num_fwd fields from an old num_retry and num_fwd.

Fixes: https://github.com/ceph/ceph/pull/45669
Fixes: https://tracker.ceph.com/issues/63288
Fixes: commit cbd7e3040208 ("ceph_fs.h: add 32 bits extended num_retry and num_fwd support")
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
(cherry picked from commit 43f32a46aa9095b19525357ba7ca215e842b4f77)