iflib: Add subinterface interrupt allocation function

The ice(4) driver will add the ability to create extra interfaces
that hang off of the base interface; to do that the driver requires
a method for the subinterface to request hardware interrupt resources
from the base interface.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D39930