Add a flag to the xfs btree cursor when using long (64bit) block pointers
instead of checking btnum == XFS_BTNUM_BMAP.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32181a
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Bill O'Donnell <billodo@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>