flow_dissector: fix false-positive __read_overflow2_field() warning

Bounds checking is unhappy that we try to copy both Ethernet
addresses but pass pointer to the first one. Luckily destination
address is the first field so pass the pointer to the entire header,
whatever.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>