macsec: dynamically allocate space for sglist

macsec: dynamically allocate space for sglist

Orabug: 25953290
CVE: CVE-2017-7477

We call skb_cow_data, which is good anyway to ensure we can actually
modify the skb as such (another error from prior). Now that we have the
number of fragments required, we can safely allocate exactly that amount
of memory.

Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 5294b83086cc1c35b4efeca03644cf9d12282e5b)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	drivers/net/macsec.c