vnet: (read) lock the vnet list while iterating it

Ensure that the vnet list cannot be modified while we're running through
it.

Reviewed by: mjg (previous version), zlei (previous version)
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42927

(cherry picked from commit bd7b2f95019e9715150c34736279805de0818d09)