Change the way IP header pointer is calculated in esp_decrypt nodes

Change the way IP header pointer is calculated in esp_decrypt nodes

The pointer to IP header was derived from l3_hdr_offset,
which would be ok, if l3_hdr_offset was valid. But it does not
have to be, so it was a bad solution. Now the previous nodes
mark whether it is a IPv6 or IPv4 packet tyle, and in esp_decrypt
we count get ip header pointer by substracting the size
of the ip header from the pointer to esp header (which lies
in front of the ip header).

Change-Id: I6d425b90931053711e8ce9126811b77ae6002a16
Signed-off-by: Szymon Sliwa <szs@semihalf.com>