skd: Reduce memory usage

Every single coherent DMA memory buffer occupies at least one page.
Reduce memory usage by switching from coherent buffers to streaming
DMA for I/O requests (struct skd_fitmsg_context) and S/G-lists
(struct fit_sg_descriptor[]).

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>