partitions/efi: Support non-standard GPT location

Support looking up GPT at a non-standard location specified by a block
device driver.

Acked-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-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/20210820004536.15791-3-digetx@gmail.com


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