xfrm interface if_id = 0 would cause xfrm policy lookup errors since
Commit 9f8550e4.

Now explicitly fail to create an xfrm interface when if_id = 0

With this commit:
 ip link add ipsec0  type xfrm dev lo  if_id 0
 Error: if_id must be non zero.

v1->v2 change:
 - add Fixes: tag

Fixes: 9f8550e4 ("xfrm: fix disable_xfrm sysctl when used on xfrm interfaces")
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Reviewed-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>