direct-io: always call ->end_io if non-NULL

direct-io: always call ->end_io if non-NULL

This way we can pass back errors to the file system, and allow for
cleanup required for all direct I/O invocations.

Also allow the ->end_io handlers to return errors on their own, so that
I/O completion errors can be passed on to the callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
(cherry-picked commit from 187372a3b9faff68ed61c291d0135e6739e0dbdf)

Orabug: 24393811
Conflicts:
    The change in the return type of the function dio_iodone_t
broke the KABI. Hence, the original function return type is wrapped
around under the flag __GENKSYMS__

Signed-off-by: Ashok Vairavan <ashok.vairavan@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>