block: Add alternative_gpt_sector() operation

Add alternative_gpt_sector() block device operation which specifies
alternative location of a GPT entry. This allows us to support Android
devices that have GPT entry at a non-standard location and can't be
repartitioned easily.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20210820004536.15791-2-digetx@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -1872,6 +1872,13 @@ struct block_device_operations {
 	char *(*devnode)(struct gendisk *disk, umode_t *mode);
 	struct module *owner;
 	const struct pr_ops *pr_ops;
+
+	/*
+	 * Special callback for probing GPT entry at a given sector.
+	 * Needed by Android devices, used by GPT scanner and MMC blk
+	 * driver.
+	 */
+	int (*alternative_gpt_sector)(struct gendisk *disk, sector_t *sector);
 };
 
 #ifdef CONFIG_COMPAT