kern/55071: Panic shortly after running X11 due to kernel diagnostic assertion "mutex_owned(&pp->pp_lock)"

[venture73%geeklan.co.uk@localhost]

>Number:         55071
>Category:       kern
>Synopsis:       Panic shortly after running X11 due to kernel diagnostic assertion "mutex_owned(&pp->pp_lock)"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Mar 14 02:00:00 +0000 2020
>Originator:     Sevan Janiyan
>Release:        HEAD
>Organization:
>Environment:
GENERIC amd64 202003130750Z
>Description:
panic: kernel diagnostic assertion mutex_owned(&pp->pp_lock)" failed for file "/home/source/ab/HEAD/src/sys/arch/x86/x86/pmap.c" line 532

bt
https://pbs.twimg.com/media/ETB8p_4XkAAMCwF?format=jpg&name=orig

This was introduced sometime since 22nd of February.
>How-To-Repeat:
Boot amd64 build, startx wait a minute
>Fix: