MFC r352350: rangelock: add rangelock_cookie_assert

A future change to posixshm to add file sealing 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.