miniflow_extract: Properly handle small IP packets.

miniflow_extract: Properly handle small IP packets.

Ethernet frames may contain padding after the IP payload.  When
parsing IP packets, check the IP total size (IPv4) or IP payload size
(IPv6) to detect the size of l2 padding.  The l2 padding size is
stored in the ofpbuf to prevent ofpbuf_pull from entering the padding,
as well as to allow ofpbuf_l4_size() to return the size of the IP
payload without the l2 padding.

This helps avoiding parsing truncated transport headers, for example.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>