drop_monitor: Split tracing enable / disable to different functions

Subsequent patches will need to enable / disable tracing based on the
configured alerting mode.

Reduce the nesting level and prepare for the introduction of this
functionality by splitting the tracing enable / disable operations into
two different functions.

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