[XFS] refactor btree validation helpers

[XFS] refactor btree validation helpers

Move the various btree validation helpers around in xfs_btree.c so that
they are close to each other and in common #ifdef DEBUG sections.

Also add a new xfs_btree_check_ptr helper to check a btree ptr that can be
either long or short form.

Split out from a bigger patch from Dave Chinner with various small changes
applied by me.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32183a

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>