nios2: page fault et.al. are *not* restartable syscalls...

make sure that ->orig_r2 is negative for everything except
the syscalls.

Fixes: 82ed08dd ("nios2: Exception handling")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>