ceph: Fix oops when handling mdsmap that decreases max_mds

(cherry picked from commit 3e8f43a089f06279c5f76a9ccd42578eebf7bfa5)

When i >= newmap->m_max_mds, ceph_mdsmap_get_addr(newmap, i) return
NULL. Passing NULL to memcmp() triggers oops.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>