rangelock: add rangelock_cookie_assert

A future change to posixshm to add file sealing (in DIFF_21391[0] and child)
will move locking out of shm_dotruncate as kern_shm_open() will require the
lock to be held across the dotruncate until the seal is actually applied.
For this, the cookie is passed into shm_dotruncate_locked which asserts
RCA_WLOCKED.

[0] Name changed to protect the innocent, hopefully, from getting autoclosed
due to this reference...

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D21628