writeback: Fix some comment errors

Signed-off-by: Wanpeng Li <liwp@linux.vnet.ibm.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>

diff --cc fs/fs-writeback.c
Simple merge

diff --cc fs/super.c
Simple merge

diff --cc fs/sync.c
Simple merge

diff --cc mm/page-writeback.c
Simple merge