nfsd: clean up svc_reserve_auth()

This is a void function attempting to return the return value from
another void function, which seems harmless but extremely weird, and
apparently makes some compilers complain.

While we're there, clean up a little (e.g. the switch statement had a
minor style problem and seemed overkill as long as there's only one
case).

Thanks to Trond for noticing this.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>