ceph: fix incorrect use of strncpy

GCC8 prints following warning:

 fs/ceph/mds_client.c:3683:2: warning: ‘strncpy’ output may be truncated
 copying 64 bytes from a string of length 64 [-Wstringop-truncation]

[ Change to strscpy() while at it. ]

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>