samples: bpf: Add BPF support for devmap_xmit tracepoint

This adds support for the devmap_xmit tracepoint, and its multi device
variant that can be used to obtain streams for each individual
net_device to net_device redirection. This is useful for decomposing
total xmit stats in xdp_monitor.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-11-memxor@gmail.com