SUNRPC: remove machine_cred field from struct auth_cred

The cred is a machine_cred iff ->principal is set, so there is no
need for the extra flag.

There is one case which deserves some
explanation. nfs4_root_machine_cred() calls rpc_lookup_machine_cred()
with a NULL principal name which results in not getting a machine
credential, but getting a root credential instead.
This appears to be what is expected of the caller, and is
clearly the result provided by both auth_unix and auth_gss
which already ignore the flag.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>