lib/ref_tracker: add printing to memory buffer

Similar to stack_(depot|trace)_snprint the patch
adds helper to printing stats to memory buffer.
It will be helpful in case of debugfs.

Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>