xfs: fix di_onlink checking for V1/V2 inodes

"KjellR" complained on IRC that an old V4 filesystem suddenly stopped
mounting after upgrading from 6.9.11 to 6.10.3, with the following splat
when trying to read the rt bitmap inode:

00000000: 49 4e 80 00 01 02 00 01 00 00 00 00 00 00 00 00  IN..............
00000010: 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00  ................
00000020: 00 00 00 00 00 00 00 00 43 d2 a9 da 21 0f d6 30  ........C...!..0
00000030: 43 d2 a9 da 21 0f d6 30 00 00 00 00 00 00 00 00  C...!..0........
00000040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
00000050: 00 00 00 02 00 00 00 00 00 00 00 04 00 00 00 00  ................
00000060: ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 00  ................
00000070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................

As Dave Chinner points out, this is a V1 inode with both di_onlink and
di_nlink set to 1 and di_flushiter == 0.  In other words, this inode was
formatted this way by mkfs and hasn't been touched since then.

Back in the old days of xfsprogs 3.2.3, I observed that libxfs_ialloc
would set di_nlink, but if the filesystem didn't have NLINK, it would
then set di_version = 1.  libxfs_iflush_int later sees the V1 inode and
copies the value of di_nlink to di_onlink without zeroing di_onlink.

Eventually this filesystem must have been upgraded to support NLINK
because 6.10 doesn't support !NLINK filesystems, which is how we tripped
over this old behavior.  The filesystem doesn't have a realtime section,
so that's why the rtbitmap inode has never been touched.

Fix this by removing the di_onlink/di_nlink checking for all V1/V2
inodes because this is a muddy mess.  The V3 inode handling code has
always supported NLINK and written di_onlink==0 so keep that check.
The removal of the V1 inode handling code when we dropped support for
!NLINK obscured this old behavior.

Reported-by: kjell.m.randa@gmail.com
Fixes: 40cb8613 ("xfs: check unused nlink fields in the ondisk inode")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/libxfs/xfs_inode_buf.c

@@ -514,12 +514,18 @@ xfs_dinode_verify(
 			return __this_address;
 	}
 
-	if (dip->di_version > 1) {
+	/*
+	 * Historical note: xfsprogs in the 3.2 era set up its incore inodes to
+	 * have di_nlink track the link count, even if the actual filesystem
+	 * only supported V1 inodes (i.e. di_onlink).  When writing out the
+	 * ondisk inode, it would set both the ondisk di_nlink and di_onlink to
+	 * the the incore di_nlink value, which is why we cannot check for
+	 * di_nlink==0 on a V1 inode.  V2/3 inodes would get written out with
+	 * di_onlink==0, so we can check that.
+	 */
+	if (dip->di_version >= 2) {
 		if (dip->di_onlink)
 			return __this_address;
-	} else {
-		if (dip->di_nlink)
-			return __this_address;
 	}
 
 	/* don't allow invalid i_size */