afs: change afs_nfs{2,3}_dispatcher signature

afs: change afs_nfs{2,3}_dispatcher signature

The fourth argument of the afs_nfs{2,3}_dispatcher functions is a pointer to a
pointer to a exportinfo structure.  However, this argument is not an output
argument, so the extra level of indirection is unnecessary.  A separate local
variable is used as an output argument to the afs_nfsclient_reqhandler call
within the dispatchers, which is not passed back to the afs_nfs{2,3}_dispatcher
caller.

In anticipation of other changes to fix warning messages, simplify the
signature of the afs_nfs{2,3}_dispatcher functions to avoid taking the address
of the exportinfo structure when calling afs_nfs{2,3}_dispatcher.

Change-Id: I6fb1a190e6aab286bfac41df783688a0be46a21f
Reviewed-on: https://gerrit.openafs.org/12988
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>