nfsd: cleanup nfsd module initialization cleanup

Handle the failure case here with something closer to the standard
kernel style.

Doesn't really matter for now, but I'd like to add a few more failure
cases, and then this'll help.

Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>