parasite: simplify syscall check

In is_required_syscall() req_sysnr is set according to the native mode
so we can check the syscall number directly against it.

No functional change expected.

Cc: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>