kdat: Relax uffd checks (v2)

v2: When uffd is present, the reported features may still be 0,
    so we need one more bool for uffd syscall itself.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>