blk_end_request: changing um (take 4)

This patch converts um to use blk_end_request interfaces.
Related 'uptodate' arguments are converted to 'error'.

As a result, the interface of internal function, ubd_end_request(),
is changed.

Cc: Jeff Dike <jdike@karaya.com>
Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>