upgrade/dumpling-x: whitelist mon scrub mismatch

A mixed dumpling/firefly cluster will have each mon independently encode
the full map.  Firefly now puts feature bits in the Incremental so that
we encode the "right" way, but dumpling doesn't have that capability
yet, and we don't plan to backport it.

Fixes: #7704
Signed-off-by: Sage Weil <sage@inktank.com>