cr-exec: check syscall's number right before injecting it

So, I made a mistake in find_syscall: it can't use arch_task_compatible
to find out in which mode the application is (native/compat).
The reason is that arch_task_compatible uses PTRACE_GETREGSET.
And at the moment of find_syscall we haven't yet seized the task.

In this patch I move syscall's number check right before injecting
a syscall, where we have parasite_ctl with all needed information
about task's mode.
This makes error-path for wrong syscall number longer and subtler
(includes curing now), but I think it's a corner-case, so as
it makes error-less path to execute_syscall shorter and without
additional ptrace syscalls, it's better.

Reported-by: Andrew Vagin <avagin@virtuozzo.com>
Cc: Andrew Vagin <avagin@virtuozzo.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>