vt(4): Fix a LOR which occurs during a call to vt_upgrade()

Reported by:	kib@
Review:		https://reviews.freebsd.org/D785
Reviewed by:	ray@
Approved by:	ray@
MFC after:	2 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271682 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f