xfrm: do not leak ESRCH to user space

I noticed that, under certain conditions, ESRCH can be leaked from the
xfrm layer to user space through sys_connect. In particular, this seems
to happen reliably when the kernel fails to resolve a template either
because the AF_KEY receive buffer being used by racoon is full or
because the SA entry we are trying to use is in XFRM_STATE_EXPIRED
state.

However, since this could be a transient issue it could be argued that
EAGAIN would be more appropriate. Besides this error code is not even
documented in the man page for sys_connect (as of man-pages 3.07).
Signed-off-by: NFernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/xfrm/xfrm_policy.c

@@ -1251,6 +1251,8 @@ xfrm_tmpl_resolve_one(struct xfrm_policy *policy, struct flowi *fl,
 				 -EINVAL : -EAGAIN);
 			xfrm_state_put(x);
 		}
+		else if (error == -ESRCH)
+			error = -EAGAIN;
 
 		if (!tmpl->optional)
 			goto fail;