[IA64] Proper handling of TLB errors from duplicate itr.d dropins

Jack Steiner noticed that duplicate TLB DTC entries do not cause a
linux panic.  See discussion:

http://www.gelato.unsw.edu.au/archives/linux-ia64/0307/6108.html

The current TLB recovery code is recovering from the duplicate itr.d
dropins, masking the underlying problem.  This change modifies
the MCA recovery code to look for the TLB check signature of the
duplicate TLB entry and panic in that case.

Signed-off-by: Russ Anderson (rja@sgi.com)
Signed-off-by: Tony Luck <tony.luck@intel.com>