loader ptblread() is broken with >512B sectors

The loader strategy() function is assuming 512B blocks, so we need to adjust
ptblread() for other sector sizes.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D12847

sys/boot/common/disk.c

@@ -88,6 +88,12 @@ ptblread(void *d, void *buf, size_t blocks, uint64_t offset)
 	dev = (struct disk_devdesc *)d;
 	od = (struct open_disk *)dev->d_opendata;
 
+	/*
+	 * The strategy function assumes the offset is in units of 512 byte
+	 * sectors. For larger sector sizes, we need to adjust the offset to
+	 * match the actual sector size.
+	 */
+	offset *= (od->sectorsize / 512);
 	/*
 	 * As the GPT backup partition is located at the end of the disk,
 	 * to avoid reading past disk end, flag bcache not to use RA.