mds: MDSMap: adjust buffer size for uint64 values with more than 5 chars

Fixes: #6620

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #761 from ceph/wip-6620

mds: MDSMap: adjust buffer size for uint64 values with more than 5 chars

Backport: dumpling, cuttlefish