SeqWait: don't do wakeups under the lock

Clippy pointed out that `drop(waiters)` didn't do anything, because
there was a misplaced ";" causing `waiters` to be a unit type `()`.

This change makes it do what was intended: the lock should be dropped
first, then the wakeups should be processed.

zenith_utils/src/seqwait.rs

@@ -63,7 +63,7 @@ impl SeqWait {
 
             // This will steal the entire waiters map.
             // When we drop it all waiters will be woken.
-            mem::take(&mut internal.waiters);
+            mem::take(&mut internal.waiters)
 
             // Drop the lock as we exit this scope.
         };