minidump: Use provided msgbuf pointer

Don't assume we are dumping the global message buffer, but use the one
provided by the state argument. While here, drop superfluous
cast to char *.

Reviewed by: markj, jhb
MFC after: 2 weeks
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D31991