This adds nowait support to the RAID10 driver. Very similar to
raid1 driver changes. It makes RAID10 driver return with EAGAIN
for situations where it could wait for eg:

  - Waiting for the barrier,
  - Reshape operation,
  - Discard operation.

wait_barrier() and regular_request_wait() fn are modified to return bool
to support error for wait barriers. They returns true in case of wait
or if wait is not required and returns false if wait was required
but not performed to support nowait.
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Vishal Verma <vverma@digitalocean.com>
Signed-off-by: Song Liu <song@kernel.org>