compel: Do not loose sign of result in compat syscall

Regs are present in unsigned format so convert them
into signed first to provide results.

In particular if memfd_create syscall failed we won't
notice -ENOMEM error but rather treat it as unsigned
hex value

 | (05.303002) Putting parasite blob into 0x7f1c6ffe0000->0xfffffff4
 | (05.303234) Putting tsock into pid 42773

Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Reviewed-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>