selftests/bpf: Implement get_hw_ring_size function to retrieve current and max interface size

Introduce a new function called get_hw_size that retrieves both the
current and maximum size of the interface and stores this information
in the 'ethtool_ringparam' structure.

Remove ethtool_channels struct from xdp_hw_metadata.c due to redefinition
error. Remove unused linux/if.h include from flow_dissector BPF test to
address CI pipeline failure.

Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20240402114529.545475-4-tushar.vyavahare@intel.com