xfs: Change how listxattr generates synthetic attributes

Instead of adding the synthesized POSIX ACL attribute names after listing all
non-synthesized attributes, generate them immediately when listing the
non-synthesized attributes.

In addition, merge xfs_xattr_put_listent and xfs_xattr_put_listent_sizes to
ensure that the list size is computed correctly; the split version was
overestimating the list size for non-root users.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: xfs@oss.sgi.com
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>