NFSD: Add nfsd4_encode_fattr4_acl_trueform

NFSD: Add nfsd4_encode_fattr4_acl_trueform

Mapping between NFSv4 ACLs and POSIX ACLs is semantically imprecise:
a client that sets an NFSv4 ACL and reads it back may see a different
ACL than it wrote. The proposed NFSv4 POSIX ACL extension introduces
the FATTR4_ACL_TRUEFORM attribute, which reports whether a file
object stores its access control permissions using NFSv4 ACLs or
POSIX ACLs.

A client aware of this extension can avoid lossy translation by
requesting and setting ACLs in their native format.

When NFSD is built with CONFIG_NFSD_V4_POSIX_ACLS, report
ACL_MODEL_POSIX_DRAFT for file objects on file systems with the
SB_POSIXACL flag set, and ACL_MODEL_NONE otherwise. Linux file
systems do not store NFSv4 ACLs natively, so ACL_MODEL_NFS4 is never
reported.

Signed-off-by: Rick Macklem <rmacklem@uoguelph.ca>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>