This fixes a locking problem noted by Tim Hockin:
    * /proc/swaps uses seq_file code, calling seq_path() with swaplock held
    * seq_path() calls d_path()
    * d_path() calls mntput() which might_sleep()

We add a new semaphore protecting insertions/removals in the set of swap
components + switch of ->start()/->stop() to the same semaphore [fixes
deadlocks] + trivial cleanup of ->next().