vfs: use vn_lock_pair to avoid establishing an ordering on mount

This fixes some of the LORs seen on mount/unmount.

Complete fix will require taking care of unmount as well.

Reviewed by: kib
Tested by: pho (previous version)
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31611