block: move lower_48_bits() to block

The function is not generally applicable enough to be included in the core
kernel header. Move it to block since it's the only subsystem using it.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20220327173316.315-1-kbusch@kernel.org


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