If bpfilter is not available return ENOPROTOOPT to fallback to netfilter.

Function request_module() returns both errors and userspace exit codes.
Just ignore them. Rechecking bpfilter_ops is enough.

Fixes: d2ba09c1 ("net: add skeleton of bpfilter kernel module")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>