xfs: use a lockref for the buffer reference count

xfs: use a lockref for the buffer reference count

The lockref structure allows incrementing/decrementing counters like
an atomic_t for the fast path, while still allowing complex slow path
operations as if the counter was protected by a lock.  The only slow
path operations that actually need to take the lock are the final
put, LRU evictions and marking a buffer stale.

Signed-off-by: Christoph Hellwig <hch@lst.de>