svcrpc: fix BUG() in svc_tcp_clear_pages

commit be1e4444 upstream.

Examination of svc_tcp_clear_pages shows that it assumes sk_tcplen is
consistent with sk_pages[] (in particular, sk_pages[n] can't be NULL if
sk_tcplen would lead us to expect n pages of data).

svc_tcp_restore_pages zeroes out sk_pages[] while leaving sk_tcplen.
This is OK, since both functions are serialized by XPT_BUSY.  However,
that means the inconsistency must be repaired before dropping XPT_BUSY.

Therefore we should be ensuring that svc_tcp_save_pages repairs the
problem before exiting svc_tcp_recv_record on error.

Symptoms were a BUG() in svc_tcp_clear_pages.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/sunrpc/svcsock.c

@@ -1136,9 +1136,9 @@ static int svc_tcp_recvfrom(struct svc_rqst *rqstp)
 	if (len >= 0)
 		svsk->sk_tcplen += len;
 	if (len != want) {
+		svc_tcp_save_pages(svsk, rqstp);
 		if (len < 0 && len != -EAGAIN)
 			goto err_other;
-		svc_tcp_save_pages(svsk, rqstp);
 		dprintk("svc: incomplete TCP record (%d of %d)\n",
 			svsk->sk_tcplen, svsk->sk_reclen);
 		goto err_noclose;