mptcp: remove redundant initialization in pm_nl_init_net()

Memory of struct pm_nl_pernet{} is allocated by kzalloc()
in setup_net()->ops_init(), so it's no need to reset counters
and zero bitmap in pm_nl_init_net().

Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>