signal/ia64: Replace FPE_FIXME with FPE_FLTUNK

Using an si_code of 0 that aliases with SI_USER is clearly the wrong
thing todo, and causes problems in interesting ways.

The newly defined FPE_FLTUNK semantically appears to fit the bill so
use it instead.

Given recent experience in this area odds are it will not
break anything.  Fixing it removes a hazard to kernel maintenance.

Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: linux-ia64@vger.kernel.org
Fixes: 987159266c45 ("Linux version 2.3.48")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>