ethernet: Copy only 6 bytes of mac address into a u64

ethernet: Copy only 6 bytes of mac address into a u64

Rather than leaving 2 bytes of junk in the upper word
of a mac address represented as a u64, zero them out.
That way later compairsons stand a chance of matching
when deleting a bridge's arp termination entries.

The volatile qualifier shouldn't be needed here, but
without it the compiler removes the clib_memcpy() at -O2.
Bad compiler.  No biscuit.

Type: fix
commit: faf22cb303b65e2a6bf8dad959d7f5ee6d031c4f

Change-Id: Iebcf35fdd421293dccbcaefadef767f7e139438e
Signed-off-by: Jon Loeliger <jdl@netgate.com>
(cherry picked from commit 6a32ce326495bfe48ebef74dfbb8a9c1cf37a530)