[IPSEC] Fix block size/MTU bugs in ESP

This patch fixes the following bugs in ESP:

* Fix transport mode MTU overestimate.  This means that the inner MTU
  is smaller than it needs be.  Worse yet, given an input MTU which
  is a multiple of 4 it will always produce an estimate which is not
  a multiple of 4.

  For example, given a standard ESP/3DES/MD5 transform and an MTU of
  1500, the resulting MTU for transport mode is 1462 when it should
  be 1464.

  The reason for this is because IP header lengths are always a multiple
  of 4 for IPv4 and 8 for IPv6.

* Ensure that the block size is at least 4.  This is required by RFC2406
  and corresponds to what the esp_output function does.  At the moment
  this only affects crypto_null as its block size is 1.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>