xfs: Add inode pin counts to traces

We don't record pin counts in inode events right now, and this makes
it difficult to track down problems related to pinning inodes. Add
the pin count to the inode trace class and add trace events for
pinning and unpinning inodes.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>