Use ip/ipv6 structures in al_eth only if they are supported

The ip/ipv6 header files are included only if the appropriate definition
exists, but the driver was missing similar checks when using the
ip and ip6_hdr structures.

If the kernel was not built with the INET or INET6 option, the driver
was preventing kernel from being built.

To fix that, the missing ifdef checks were added to the driver.

PR: Bug 230886
Submitted by: Michal Krawczyk <mk@semihalf.com>
Reported by: O. Hartmann
Approved by: re (gjb)
Obtained from: Semihalf
MFC after: 1 week
Sponsored by: Amazon, Inc.