xfsprogs: add supported file attributes to xfs.5 manpage

xfsprogs: add supported file attributes to xfs.5 manpage

The chattr(1) manpage suffers from the same problems mount(1) had:
many options listed, not kept up to date for various filesystems.

I've submitted a manpage update for chattr(1) which says to refer to
filesystem-specific manpages for supported attributes; this patch
updates xfs(5) to list the attributes supported by xfs.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>