unionfs: lock newly-created vnodes before calling insmntque()

This fixes an insta-panic when attempting to use unionfs with
DEBUG_VFS_LOCKS. Note that unionfs still has a long way to
go before it's generally stable or usable.

Reviewed by: kib (prior version), markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D31917