afs: fix missing afs_nfs3_dispatcher return value

afs: fix missing afs_nfs3_dispatcher return value

Fix a missing early return value in the function afs_nfs3_dispatcher.  All
callers check the return code of afs_nfs3_dispatcher and interpret values
greater that 1 to be errors. Return 3 as an error code for this code path,
which is the next available error code in afs_nfs3_dispatcher.

This commit fixes the following Solaris Studio warning message:

    ... warning: function expects to return value: afs_nfs3_dispatcher

Change-Id: I47b545bd57a46c03006b9f031da3647c8a530377
Reviewed-on: https://gerrit.openafs.org/12987
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>