xfs: bump up nr_to_write in xfs_vm_writepage

VM calculation for nr_to_write seems off.  Bump it way
up, this gets simple streaming writes zippy again.
To be reviewed again after Jens' writeback changes.

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Cc: Chris Mason <chris.mason@oracle.com>
Reviewed-by: Felix Blyakher <felixb@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>