libnvme: Export controller SGL support in nvme_ctrlr_stat.

libnvme has routines to convert SGL requests into PRP ones,
but they can only go so far, so some requests are truly
SGL-only.

src/add-ons/kernel/drivers/disk/nvme/libnvme/nvme.c

@@ -286,6 +286,8 @@ int nvme_ctrlr_stat(struct nvme_ctrlr *ctrlr, struct nvme_ctrlr_stat *cstat)
 	/* Maximum transfer size */
 	cstat->max_xfer_size = ctrlr->max_xfer_size;
 
+	cstat->sgl_supported = (ctrlr->flags & NVME_CTRLR_SGL_SUPPORTED);
+
 	memcpy(&cstat->features, &ctrlr->feature_supported,
 	       sizeof(ctrlr->feature_supported));
 	memcpy(&cstat->log_pages, &ctrlr->log_page_supported,

src/add-ons/kernel/drivers/disk/nvme/libnvme/nvme.h

@@ -301,6 +301,14 @@ struct nvme_ctrlr_stat {
 	 */
 	bool			log_pages[256];
 
+	/**
+	 * Whether SGL is supported by the controller.
+	 *
+	 * Note that this does not mean all SGL requests will fail;
+	 * many are convertible into standard (PRP) requests by libnvme.
+	 */
+	bool			sgl_supported;
+
 	/**
 	 * All the features supported.
 	 */