Use new pg_authenticate method to simplify nfs4 callback authentication.

This also has the effect of changing the error return from rejectedcred to
badcred.  I believe the change is correct.
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>