ubd: cleanup completion path

ubd had its own block request partial completion mechanism, which is
unnecessary as block layer already does it.  Kill ubd_end_request()
and ubd_finish() and replace them with direct call to
blk_end_request().

[ Impact: cleanup ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>