do_wait: Add missing tasklist unlocking in error paths.

[Dave Jones <davej@redhat.com>]

It looks like both 64a16caf5, and 98abed020 both introduced error paths to do_wait
where we miss unlocking the tasklist. 

Spotted with coverity.

Signed-off-by: Dave Jones <davej@fedoraproject.org>

diff --git a/kernel/exit.c b/kernel/exit.c
index a949819..27004a6 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1526,13 +1526,15 @@ repeat:
 	tsk = current;
 	do {
 		retval = do_wait_thread(wo, tsk);
-		if (retval)
+		if (retval) {
+			read_unlock(&tasklist_lock);
 			goto end;
-
+		}
 		retval = ptrace_do_wait(wo, tsk);
-		if (retval)
+		if (retval) {
+			read_unlock(&tasklist_lock);
 			goto end;
-
+		}
 		if (wo->wo_flags & __WNOTHREAD)
 			break;
 	} while_each_thread(current, tsk);