nfsd4: implement minimal SP4_MACH_CRED

Do a minimal SP4_MACH_CRED implementation suggested by Trond, ignoring
the client-provided spo_must_* arrays and just enforcing credential
checks for the minimum required operations.

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

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

diff --cc fs/nfsd/nfs4xdr.c
Simple merge

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