nfsd4: enforce per-client sessions/no-sessions distinction

Something like creating a client with setclientid and then trying to
confirm it with create_session may not crash the server, but I'm not
completely positive of that, and in any case it's obviously bad client
behavior.

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

diff --cc fs/nfsd/nfs4proc.c
Simple merge

diff --cc fs/nfsd/nfs4state.c
Simple merge

diff --cc fs/nfsd/state.h
Simple merge