lib: objagg: add root count to stats

Count number of roots and add it to stats. It is handy for the library
user to have this stats available as it can act upon it without
counting roots itself.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>