loadpin: stop using bdevname

Use the %pg format specifier to save on stack consuption and code size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220512062014.1826835-1-hch@lst.de