block: make blk_end_request_cur() return bool

In the process of mindlessly copying [__]blk_end_request_all(),
[__]blk_end_request_cur() ended up returning void even though they're
partial completion functions.  Fix it.

[ Impact: fix braindead API ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>