UPSTREAM: 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.

Reviewed-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 8a2d9f00d502e6ef68c6d52f0863856040ddd2db)
Change-Id: I7ab430a7294fa9644512f0a32245646944446213
Signed-off-by: zhaoyuenan <amktiao030215@gmail.com>