btrfs: add a node counter to each of the rbtrees

This patch adds counters to each of the rbtrees so that we can tell
how large they are growing for a given workload.  These counters
will be exported by tracepoints in the next patch.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>