xfs: use GFP_NOFS for page cache allocation

Avoid a lockdep warning by preventing page cache allocation from
recursing back into the filesystem during memory reclaim.

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