[IA64] Avoid a rare deadlock during unwind

[IA64] Avoid a rare deadlock during unwind

There is a rare deadlock condition during unwind script creation.  If
build_script() is interrupted in the middle of creating the script, it
holds the script write lock.  If the interrupt handler needs to call
unwind for some failure condition, unwind will try to read the
incomplete script and will deadlock on the script lock.

The fix is to disable interrupts while building the script, so
interrupt handlers never see partial scripts.

Promoting spin_lock_irqsave() from script_new() to find_save_locs()
changes the indentation, so the patch looks bigger than it really is.

Signed-off-by: Keith Owens <kaos@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>