NFSD: Add a Kconfig setting to enable support for NFSv4 POSIX ACLs

NFSD: Add a Kconfig setting to enable support for NFSv4 POSIX ACLs

A new IETF draft extends NFSv4.2 with POSIX ACL attributes:

  https://www.ietf.org/archive/id/draft-ietf-nfsv4-posix-acls-00.txt

This draft has not yet been ratified. A build-time configuration
option allows developers and distributors to decide whether to
expose this experimental protocol extension to NFSv4 clients. The
option is disabled by default to prevent unintended deployment of
potentially unstable protocol features in production environments.

This approach mirrors the existing NFSD_V4_DELEG_TIMESTAMPS option,
which gates another experimental NFSv4 extension based on an
unratified IETF draft.

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