xfrm: fix disable_xfrm sysctl when used on xfrm interfaces

The disable_xfrm flag signals that xfrm should not be performed during
routing towards a device before reaching device xmit.

For xfrm interfaces this is usually desired as they perform the outbound
policy lookup as part of their xmit using their if_id.

Before this change enabling this flag on xfrm interfaces prevented them
from xmitting as xfrm_lookup_with_ifid() would not perform a policy lookup
in case the original dst had the DST_NOXFRM flag.

This optimization is incorrect when the lookup is done by the xfrm
interface xmit logic.

Fix by performing policy lookup when invoked by xfrmi as if_id != 0.

Similarly it's unlikely for the 'no policy exists on net' check to yield
any performance benefits when invoked from xfrmi.

Fixes: f203b76d

 ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>