[XFS] add get_maxrecs btree operation

Factor xfs_btree_maxrecs into a per-btree operation.

The get_maxrecs method is based on a patch from Dave Chinner.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32188a

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>