xfs: Add log level to assertion printk

I received a ppc64 bug report involving xfs but the assertion was
filtered out by the console log level. Use KERN_CRIT to ensure it
makes it out.

Signed-off-by: Anton Blanchard <anton@samba.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>