blk-zoned: Move code from disk_zone_wplug_add_bio() into its caller

blk-zoned: Move code from disk_zone_wplug_add_bio() into its caller

Move the following code into the only caller of disk_zone_wplug_add_bio():
 - The code for clearing the REQ_NOWAIT flag.
 - The code that sets the BLK_ZONE_WPLUG_PLUGGED flag.
 - The disk_zone_wplug_schedule_bio_work() call.

This patch moves all code that is related to REQ_NOWAIT or to bio
scheduling into a single function. Additionally, the 'schedule_bio_work'
variable is removed. No functionality has been changed.

Cc: Damien Le Moal <dlemoal@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlmoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>