linuxkpi: Add __var_waitqueue

This returns the wait queue based on the object but in LinuxKPI
we only have one waitqueue for this.

Reviewed by: hselasky, bz
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D34250

(cherry picked from commit cb15ed7da2be02b108d732e41bb6e68febf93f90)