net: ipv4: Use kfree_sensitive instead of kfree

key might contain private part of the key, so better use
kfree_sensitive to free it.

Fixes: 38320c70

 ("[IPSEC]: Use crypto_aead and authenc in ESP")
Signed-off-by: Wang Ming <machel@vivo.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>