NFSD: Add nfsd4_encode_fattr4_posix_default_acl

NFSD: Add nfsd4_encode_fattr4_posix_default_acl

The POSIX ACL extension to NFSv4 defines FATTR4_POSIX_DEFAULT_ACL
for retrieving a directory's default ACL. This patch adds the
XDR encoder for that attribute.

For directories, the default ACL is retrieved via get_inode_acl()
and each entry is encoded as a posixace4: tag type, permission
bits, and principal name (empty for structural entries like
USER_OBJ/GROUP_OBJ/MASK/OTHER, resolved via idmapping for
USER/GROUP entries).

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