mds,messages: silence -Wclass-memaccess warnings

mds,messages: silence -Wclass-memaccess warnings

this change silences warnings like:

warning: ‘void* memset(void*, int, size_t)’ writing to an object of
non-trivial type ‘struct uuid_d’; use assignment instead [-Wcla
ss-memaccess]
   memset(&z.cluster_fsid, 1, sizeof(z.cluster_fsid));
                                                    ^

Signed-off-by: Kefu Chai <kchai@redhat.com>