wmistat: don't restrict reading to the pid that opened the fd.

PR: 273405
MFC after: 1 week
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D41621

(cherry picked from commit 28977cb9bc15a7f1fb7d28e14cd03f8dafd4e155)

Approved by: re (gjb)