Make the existing bmap btree tracing generic so that it applies to all
btree types.

Some fragments lifted from a patch by Dave Chinner.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32187a
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>