kauth: ka_CellToRealm's "realm" parameter cannot be null

kauth: ka_CellToRealm's "realm" parameter cannot be null

Annotate ka_CellToRealm with AFS_NONNULL to indicate that its
"realm" parameter cannot be null; it does not make sense to call
this routine without this parameter.  (The static analyzer inlines
the call to ka_ExpandCell and concludes that "realm" might be null;
the annotation will prevent that and avoid a false positive.)

Change-Id: Id7fac19cf4eb4bdb1decde81d951b8bcd96fce71
Reviewed-on: http://gerrit.openafs.org/7865
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>