xfrm: Add file to document IPsec corner case

Here documents known IPsec corner cases which need to be keep in mind when

deploy various IPsec configuration in real world production environment.

1. IPcomp: Small IP packet won't get compressed at sender, and failed on

	   policy check on receiver.

Quote from RFC3173:

2.2. Non-Expansion Policy

   If the total size of a compressed payload and the IPComp header, as

   defined in section 3, is not smaller than the size of the original

   payload, the IP datagram MUST be sent in the original non-compressed

   form.  To clarify: If an IP datagram is sent non-compressed, no

   IPComp header is added to the datagram.  This policy ensures saving

   the decompression processing cycles and avoiding incurring IP

   datagram fragmentation when the expanded datagram is larger than the

   MTU.

   Small IP datagrams are likely to expand as a result of compression.

   Therefore, a numeric threshold should be applied before compression,

   where IP datagrams of size smaller than the threshold are sent in the

   original form without attempting compression.  The numeric threshold

   is implementation dependent.

Current IPComp implementation is indeed by the book, while as in practice

when sending non-compressed packet to the peer(whether or not packet len

is smaller than the threshold or the compressed len is large than original

packet len), the packet is dropped when checking the policy as this packet

matches the selector but not coming from any XFRM layer, i.e., with no

security path. Such naked packet will not eventually make it to upper layer.

The result is much more wired to the user when ping peer with different

payload length.

One workaround is try to set "level use" for each policy if user observed

above scenario. The consequence of doing so is small packet(uncompressed)

will skip policy checking on receiver side.