f2fs: introduce f2fs_cp_error for readability

This patch adds f2fs_cp_error for readability.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --cc fs/f2fs/checkpoint.c
Simple merge

diff --cc fs/f2fs/f2fs.h
Simple merge

diff --cc fs/f2fs/gc.c
Simple merge

diff --cc fs/f2fs/super.c
Simple merge