samples/bpf: Fix xdp_redirect_map egress devmap prog

LLVM compiler optimized out the memcpy in xdp_redirect_map_egress,
which caused the Ethernet source MAC-addr to always be zero
when enabling the devmap egress prog via cmdline --load-egress.

Issue observed with LLVM version 14.0.0
 - Shipped with Fedora 36 on target: x86_64-redhat-linux-gnu.

In verbose mode print the source MAC-addr in case xdp_devmap_attached
mode is used.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/165754826292.575614.5636444052787717159.stgit@firesoul