nfsd4: fix error return in non-matching-creds case

Note CLID_INUSE is for the case where two clients are trying to use the
same client-provided long-form client identifiers.  But what we're
looking at here is the server-returned shorthand client id--if those
clash there's a bug somewhere.

Fix the error return, pull the check out into common code, and do the
check unconditionally in all cases.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>