nvme: enforce 64bit offset for nvme_get_log_ext fn

Compiling on 32 bits system produces a warning for the shift width
when shifting 32 bit integer with 64bit integer.

Make sure that offset always is 64bit, and use macros for retrieving
lower and upper bits of the offset.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>