xfs: use separate btree cursor cache for each btree type

xfs: use separate btree cursor cache for each btree type

Source kernel commit: 9fa47bdcd33b117599e9ee3f2e315cb47939ac2d

Now that we have the infrastructure to track the max possible height of
each btree type, we can create a separate slab cache for cursors of each
type of btree.  For smaller indices like the free space btrees, this
means that we can pack more cursors into a slab page, improving slab
utilization.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>