vlserver: Rationalise multi homed host processing

vlserver: Rationalise multi homed host processing

The same code for getting extent structures for multi homed hosts
was scattered throughout vlprocs.c, sometimes with error handling,
and sometimes without. Rationalise all of this into a pair of
static inline functions, which do all of the hard work.

Change-Id: Ib0f55e5b5f455b07753409ebff2a5cd0ca188cda
Reviewed-on: http://gerrit.openafs.org/4774
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>