Don't include DMAR map entry zone items in kernel dumps.

Such items may be allocated in the I/O path used by the dumper,
potentially causing the dump to fail. Since there is some precedent
in the DMAR driver for avoiding this problem using _NODUMP, apply
this workaround to the zone as well.

Reported and tested by: mmacy
Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14422