This patch provides support of TFC padding in ESP packets as specified by [[ https://tools.ietf.org/html/rfc4303#section-2.7 | RFC 4303 ]]
Padding is added between payload and cipher padding to force packet size
To control TFC padding length, a new SADB extension structure is defined (struct sadb_x_sa_tfc_length, SADB_X_EXT_SA_TFC_LENGTH)
Received packets with TFC padding is already supported as it doesn't need any specific code