[XFS] Make xfs_btree_check_ptr() debug-only code.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32224a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>