Don't pass curthread to sbreserve_locked() in tcp_do_segment(), as the

netisr or ithread's socket buffer size limit is not the right limit to
use.  Instead, pass NULL as the other two calls to sbreserve_locked()
in the TCP input path (tcp_mss()) do.

In practice, this is a no-op, as ithreads and the netisr run without a
process limit on socket buffer use, and a NULL thread pointer leads to
not using the process's limit, if any.  However, if tcp_input() is
called in other contexts that do have limits, this may prevent the
incorrect limit from being used.

MFC after:	3 days

sys/netinet/tcp_input.c

@@ -1294,7 +1294,7 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 				 */
 				if (newsize)
 					if (!sbreserve_locked(&so->so_rcv,
-					    newsize, so, curthread))
+					    newsize, so, NULL))
 						so->so_rcv.sb_flags &= ~SB_AUTOSIZE;
 				m_adj(m, drop_hdrlen);	/* delayed header drop */
 				sbappendstream_locked(&so->so_rcv, m);