skd: Move skd_free_sg_list() up

Issue a warning if a NULL argument is passed to skd_free_sg_list().
Move this function up to make the blk-mq conversion patch easier
to read.

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>