signal/ia64: Document a conflict with SI_USER with SIGFPE

Setting si_code to __SI_FAULT results in a userspace seeing
an si_code of 0.  This is the same si_code as SI_USER.  Posix
and common sense requires that SI_USER not be a signal specific
si_code.  As such this use of 0 for the si_code is a pretty
horribly broken ABI.

Given that ia64 is on it's last legs I don't know that it is worth
fixing this, but it is worth documenting what is going on so that
no one decides to copy this bad decision.

This was introduced in 2.3.51 so this mess has had a long time for
people to be able to start depending on it.

Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: linux-ia64@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>