vmm: take exclusive mem_segs_lock when (un)assigning ppt dev

PR: 268744
Reported by: mmatalka@gmail.com
Reviewed by: corvink, markj, jhb
Fixes: 67b69e76e8ee ("vmm: Use an sx lock to protect the memory map.")
Differential Revision: https://reviews.freebsd.org/D37962

(cherry picked from commit ccf32a68f821c5c724fb9a5b4b9576925122292f)