dpdk-cryptodev: sync mbuf length with corresponding vlib buffer

When vlib buffer is processed on vnet side its length is corrected by
cipher padding and icv_sz. These changes need to be reflected in
the mbuf internals.

Type: fix
Signed-off-by: Piotr Bronowski <piotrx.bronowski@intel.com>
Change-Id: I0aa03f67f556dfc8f9a577ca1967210527221e02