net: drop_monitor: fix the value of maxattr

[ Upstream commit d323e92cc3f4edd943610557c9ea1bb4bb5056e8 ]

maxattr in genl_family should be used to save the max attribute
type, but not the max command type. Drop monitor doesn't support
any attributes, so we should leave it as zero.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>