splice: Record some statistics

Add a proc file to export some statistics for debugging purposes.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Dave Chinner <david@fromorbit.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Jens Axboe <axboe@kernel.dk>
cc: linux-fsdevel@vger.kernel.org