savecore: fold maxdumps check into getbounds()

So that new callers of getbounds() don't need to duplicate it.

Reviewed by: markj
MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34783

(cherry picked from commit 4243d307d397dedb2161499742288450fd311e1b)