parisc/unwind: use copy_from_kernel_nofault()

I have no idea why get_user() is used there, but we're unwinding the
kernel stack, so we should use copy_from_kernel_nofault().

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>