espintcp: handle short messages instead of breaking the encap socket

Currently, short messages (less than 4 bytes after the length header)
will break the stream of messages. This is unnecessary, since we can
still parse messages even if they're too short to contain any usable
data. This is also bogus, as keepalive messages (a single 0xff byte),
though not needed with TCP encapsulation, should be allowed.

This patch changes the stream parser so that short messages are
accepted and dropped in the kernel. Messages that contain a valid SPI
or non-ESP header are processed as before.

Fixes: e27cca96 ("xfrm: add espintcp (RFC 8229)")
Reported-by: Andrew Cagney <cagney@libreswan.org>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>