[IA64] ia32_signal.c: erroneous use of memset/memcpy

Found by Alexander Nyberg, improved by Bjorn Helgaas.

- Fix the incorrect argument to sizeof()
- looks like memcpy() code pass was dervived from code that used
  copy_from_user().  But in this case we are doing to kernel space
  to kernel space copy, so memcpy is the right routine, but it
  doesn't return an error code.
 
Signed-off-by: Arun Sharma <arun.sharma@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>