if_vlan locking fixes

[Matt Joras]

Hello,

At Isilon we end up creating/destroying vlan interfaces a lot more
than users typically do. Unfortunately this lead to a myriad of panics
due to locking insufficiencies in if_vlan. I've fixed these internally
and I've submitted a review of the changes. I would appreciate if
anyone would like to review them.

https://reviews.freebsd.org/D5825

The essence of the changes is to make the global vlan lock an
rmlock(9) and expand its scope to synchronize reading/using ifvlans
with destruction events as well as ensure exlusivity in certain
configuration paths.

Thanks,
Matt Joras