drop_monitor: Move per-CPU data init/fini to separate functions

Currently drop monitor only reports software drops to user space, but
subsequent patches are going to add support for hardware drops.

Like software drops, the per-CPU data of hardware drops needs to be
initialized and de-initialized upon module initialization and exit. To
avoid code duplication, break this code into separate functions, so that
these could be re-used for hardware drops.

No functional changes intended.

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