ip xfrm: support setting/printing XFRMA_IF_ID attribute in states/policies

ip xfrm: support setting/printing XFRMA_IF_ID attribute in states/policies

The XFRMA_IF_ID attribute is set in policies/states for them to be
associated with an XFRM interface (4.19+).

Add support for setting / displaying this attribute.

Note that 0 is a valid value therefore set XFRMA_IF_ID if any value
was provided in command line.

Tested-by: Antony Antony <antony@phenome.org>
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>