ia64: annotate a switch fallthrough in ia64_do_signal

Also reindent the switch statement to use the normal kernel style
while at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/20190812065524.19959-2-hch@lst.de


Signed-off-by: Tony Luck <tony.luck@intel.com>