[XFS] add new btree statistics

From: Dave Chinner <dgc@sgi.com>

Introduce statistics coverage of all the btrees and cover all the btree
operations, not just some.

Invaluable for determining test code coverage of all the btree
operations....

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32184a

Signed-off-by: David Chinner <david@fromorbit.com>
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>