silence l2fib_init() invalid read of size 2 found by address-sanitizer

l2fib_make_key() casts and reads the input 6-Bytes mac_address as u64,
therefore if the mac_address is declared with 6 Bytes only, address-sanitizer
rightly triggers an invalid read on the last two Bytes.

However, l2fib_make_key() does a 16 bits shift to discard those 2 values,
therefore, this invalid read is of no consequence (and so can be silenced safely).

Change-Id: I38646fe60073093d25cdf135185d4c96136d55d0
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>