IOPRIO_PRIO_DATA in the new kernel version includes level and hint,
So Macro IOPRIO_PRIO_LEVEL is more accurate to get ckpt thread
ioprio data/level, and it is also consisten with the way setting
ckpt thread ioprio by IOPRIO_PRIO_VALUE(class, data/level).
Besides, change variable name from "data" to "level" for more readable.
Signed-off-by: Zhiguo Niu <zhiguo.niu@xxxxxxxxxx>