fs: add an iopoll method to struct file_operations

fs: add an iopoll method to struct file_operations

This new methods is used to explicitly poll for I/O completion for an
iocb.  It must be called for any iocb submitted asynchronously (that
is with a non-null ki_complete) which has the IOCB_HIPRI flag set.

The method is assisted by a new ki_cookie field in struct iocb to store
the polling cookie.

Reviewed-by: Hannes Reinecke <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>