Fix a UDP tunneling issue with rack. Basically there are two
issues.
A) Not enough hdrlen was being calculated when a UDP tunnel is

in place.

and
B) Not enough memory is allocated in racks fsb. We need to

overbook the fsb to include a udphdr just in case.

Submitted by: Peter Lei
Reviewed by: Michael Tuexen
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D30157