RO mode is the only one that requires a locked output function.  So
it's easier to move the lock into that function rather than requiring
everyone else to run under the lock.

In particular, this allows us to move the size check into the output
function without causing a potential dead-lock should the ICMP error
somehow hit the same SA on transmission.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>