fix gcc 5.4 warning: argument to 'sizeof' in 'memcpy' call is the same expression...

fix gcc 5.4 warning: argument to 'sizeof' in 'memcpy' call is the same expression as the destination

warning translates as an invalid write :
sizeof(u8* b_dmac) == 8 != sizeof(eth_hdr->dst_address) == 6

~/vpp/build-data/../src/vnet/l2/l2_vtr.c: In function 'l2pbb_get':
~/vpp/build-data/../src/vnet/l2/l2_vtr.c:734:63: error: argument to 'sizeof' in 'memcpy' call is the same expression as the destination;
        did you mean to provide an explicit length?  [-Werror=sizeof-pointer-memaccess]
~/vpp/build-data/../src/vnet/l2/l2_vtr.c:736:63: error: argument to 'sizeof' in 'memcpy' call is the same expression as the destination;
        did you mean to provide an explicit length?  [-Werror=sizeof-pointer-memaccess]

update l2pbb_get to take an ethernet header instead of two u8* pointers
for source and dest mac addresses.

Change-Id: Ifcf1319a9e22614d57682f940e10f0420dc6fb8c
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>