"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>