md: raid1 add nowait support

This adds nowait support to the RAID1 driver. It makes RAID1 driver
return with EAGAIN for situations where it could wait for eg:

  - Waiting for the barrier,

wait_barrier() fn is modified to return bool to support error for
wait barriers. It 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>