[NET]: Fix crash in dev_mc_sync()/dev_mc_unsync()

This patch fixes a crash that may occur when the routine dev_mc_sync()
deletes an address from the list it is currently going through. It
saves the pointer to the next element before deleting the current one.
The problem may also exist in dev_mc_unsync().

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>