skd: Rework request failing code path

Move the skd_fail_all_pending() call out of skd_request_fn_not_online()
such that this function can be reused in the blk-mq code path.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>