vlserver: Make space in extent block explicit

vlserver: Make space in extent block explicit

The address entry side of the union within the extent addr block
actually has a significant amount of free space. It looks as though
the original author assumed that a UUID required 16 32-bit words,
rather than 16 octets, and sized the structure to match.

Make the free space within the structure explicit, so that it can
be used for future expansion

Verified with
  gdb vlserver
    (gdb) print sizeof(struct extentaddr)
    $1 = 128

Change-Id: If67f669fb298763f054ce82447a4513c59ef7526
Reviewed-on: http://gerrit.openafs.org/4775
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>