xfs: don't warn about page discards on shutdown

If we are doing a forced shutdown, we can get lots of noise about
delalloc pages being discarded. This is happens by design during a
forced shutdown, so don't spam the logs with these messages.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>