NFSD: Add nfsd4_encode_fattr4_posix_access_acl

NFSD: Add nfsd4_encode_fattr4_posix_access_acl

The POSIX ACL extension to NFSv4 defines FATTR4_POSIX_ACCESS_ACL
for retrieving the access ACL of a file or directory. This patch
adds the XDR encoder for that attribute.

The access ACL is retrieved via get_inode_acl(). If the filesystem
provides no explicit access ACL, one is synthesized from the file
mode via posix_acl_from_mode(). Each entry is encoded as a
posixace4: tag type, permission bits, and principal name (empty
for structural entries, resolved via idmapping for USER/GROUP
entries).

Unlike the default ACL encoder which applies only to directories,
this encoder handles all inode types and ensures an access ACL is
always available through mode-based synthesis when needed.

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