[NET] slip: Replace kmalloc() + memset() pairs with the appropriate kzalloc() calls

This patch replace kmalloc() + memset() pairs with the appropriate
kzalloc().

Signed-off-by: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>