watch_queue: Free the alloc bitmap when the watch_queue is torn down

Free the watch_queue note allocation bitmap when the watch_queue is
destroyed.

Fixes: c73be61c

 ("pipe: Add general notification queue support")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>