FROMGIT: f2fs: set highest IO priority for checkpoint thread

The checkpoint is the top priority thread which can stop all the filesystem
operations. Let's make it RT priority.

Bug: 402202167
Bug: 390504516
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Link: https://lore.kernel.org/linux-f2fs-devel/20250303221730.1284822-1-jaegeuk@kernel.org/T/#u

(cherry picked from commit 8a2d9f00
https: //git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git/ dev)
Change-Id: Ife3b87ecff0ad4b8db805c1b15227f3afcd4fbd1
(cherry picked from commit 50eddb3f)