kill vn_ioerror

There's just one caller of this helper, and it's much cleaner to just merge
the xfs_do_force_shutdown call into it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>