pacific: fsmap: switch to using iterator based loop

[vshankar]

https://tracker.ceph.com/issues/57194

Segfault was triggered by the fs/mirror test.
The problem is in potential undefined behaviour
after erasing element and iterator invalidation in FSMap::filter().
Fixed by explicit iterator increment.

Fixes: https://tracker.ceph.com/issues/55134

Signed-off-by: Aliaksei Makarau aliaksei.makarau@ibm.com
(cherry picked from commit 0933314)

Conflicts:
src/mds/FSMap.h

Conflict due to use of std::string_view vs string_view.

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