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>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 0e8182edd850f061421777988974efbaa3575b9f)

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Merge commit was marked with Backport tags for dumpling and emperor, but it appears to have fallen through the cracks.
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