f2fs: use new ioprio Macro to get ckpt thread ioprio level

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@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>