f2fs: fix to update dir's i_pino during cross_rename

As Eric reported:

RENAME_EXCHANGE support was just added to fsstress in xfstests:

	commit 65dfd40a97b6bbbd2a22538977bab355c5bc0f06
	Author: kaixuxia <xiakaixu1987@gmail.com>
	Date:   Thu Oct 31 14:41:48 2019 +0800

	    fsstress: add EXCHANGE renameat2 support

This is causing xfstest generic/579 to fail due to fsck.f2fs reporting errors.
I'm not sure what the problem is, but it still happens even with all the
fs-verity stuff in the test commented out, so that the test just runs fsstress.

generic/579 23s ... 	[10:02:25]
[    7.745370] run fstests generic/579 at 2019-11-04 10:02:25
_check_generic_filesystem: filesystem on /dev/vdc is inconsistent
(see /results/f2fs/results-default/generic/579.full for details)
 [10:02:47]
Ran: generic/579
Failures: generic/579
Failed 1 of 1 tests
Xunit report: /results/f2fs/results-default/result.xml

Here's the contents of 579.full:

_check_generic_filesystem: filesystem on /dev/vdc is inconsistent
*** fsck.f2fs output ***
[ASSERT] (__chk_dots_dentries:1378)  --> Bad inode number[0x24] for '..', parent parent ino is [0xd10]

The root cause is that we forgot to update directory's i_pino during
cross_rename, fix it.

Fixes: 32f9bc25 ("f2fs: support ->rename2()")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Tested-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>