net: macsec: use skb_ensure_writable_head_tail to expand the skb

Use skb_ensure_writable_head_tail to expand the skb if needed instead of
reimplementing a similar operation.

Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>