block: Add alternative_gpt_sector() operation

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.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>