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 <fernando@oss.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>