Comment # 12 on bug 910336 from Jan Kara

OK, I think I've found the culprit of the _xfs_buf_ioapply() bug and Dave
Chinner promptly fixed it. Can you run the test kernel from
http://beta.suse.com/private/jack/910336/ please? It should soon appear there.

BTW, there's a chance the problem was triggered by a kernel update where some
kernel modules were unlinked & open when shutting down the root filesystem but
still it's worth trying whether this kernel makes things better for you.