icache: Clear fetch failed flag on flush

This fixes a bug where a load that results in a DSI, if it is placed
near the end of a page and the following page isn't mapped, can
result in the core starting to take the DSI but then jumping off to
the ISI vector.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

icache.vhdl

@@ -595,6 +595,7 @@ begin
                 r.hit_valid <= '0';
                 r.stalled_hit <= '0';
                 r.stalled_way <= to_unsigned(0, WAY_BITS);
+                r.fetch_failed <= '0';
             elsif stall_in = '1' then
                 if r.state = CLR_TAG then
                     r.stalled_hit <= '0';