espintcp: recv() should return 0 when the peer socket is closed

man 2 recv says:

    RETURN VALUE

    When a stream socket peer has performed an orderly shutdown, the
    return value will be 0 (the traditional "end-of-file" return).

Currently, this works for blocking reads, but non-blocking reads will
return -EAGAIN. This patch overwrites that return value when the peer
won't send us any more data.

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