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>
(cherry picked from commit 0e8182edd850f061421777988974efbaa3575b9f)
Merge pull request #1129 from ceph/wip-dumpling-backport-6620

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