minidump: De-duplicate is_dumpable()

minidump: De-duplicate is_dumpable()

The function is identical in each minidump implementation, so move it to
vm_phys.c. The only slight exception is powerpc where the function was
public, for use in moea64_scan_pmap().

Reviewed by:	kib, markj, imp (earlier version)
MFC after:	2 weeks
Sponsored by:	Juniper Networks, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D31884