drop_monitor: allow more events per second

It seems there is a logic error in trace_drop_common(), since we store
only 64 drops, even if they are from same location.

This fix is a one liner, but we probably need more work to avoid useless
atomic dec/inc

Now I can watch 1 Mpps drops through dropwatch...

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>