f2fs: remove unneeded f2fs_cp_error() in f2fs_create_whiteout()

f2fs_rename() has checked CP_ERROR_FLAG, so remove redundant check
in f2fs_create_whiteout().

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