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: Fernando Luis Vazquez Cao <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/xfrm/xfrm_policy.c

···
 				 -EINVAL : -EAGAIN);
 			xfrm_state_put(x);
 		}
+		else if (error == -ESRCH)
+			error = -EAGAIN;
 
 		if (!tmpl->optional)
 			goto fail;