xfrm: interface: rename xfrm_interface.c to xfrm_interface_core.c

This change allows adding additional files to the xfrm_interface module.

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Link: https://lore.kernel.org/r/20221203084659.1837829-2-eyal.birger@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>