f2fs: fix to update time in lazytime mode

generic/018 reports an inconsistent status of atime, the
testcase is as below:
- open file with O_SYNC
- write file to construct fraged space
- calc md5 of file
- record {a,c,m}time
- defrag file --- do nothing
- umount & mount
- check {a,c,m}time

The root cause is, as f2fs enables lazytime by default, atime
update will dirty vfs inode, rather than dirtying f2fs inode (by set
with FI_DIRTY_INODE), so later f2fs_write_inode() called from VFS will
fail to update inode page due to our skip:

f2fs_write_inode()
	if (is_inode_flag_set(inode, FI_DIRTY_INODE))
		return 0;

So eventually, after evict(), we lose last atime for ever.

To fix this issue, we need to check whether {a,c,m,cr}time is
consistent in between inode cache and inode page, and only skip
f2fs_update_inode() if f2fs inode is not dirty and time is
consistent as well.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/inode.c

@@ -615,7 +615,11 @@ int f2fs_write_inode(struct inode *inode, struct writeback_control *wbc)
 			inode->i_ino == F2FS_META_INO(sbi))
 		return 0;
 
-	if (!is_inode_flag_set(inode, FI_DIRTY_INODE))
+	/*
+	 * atime could be updated without dirtying f2fs inode in lazytime mode
+	 */
+	if (f2fs_is_time_consistent(inode) &&
+		!is_inode_flag_set(inode, FI_DIRTY_INODE))
 		return 0;
 
 	if (!f2fs_is_checkpoint_ready(sbi))