netfilter: Simply the tests for enabling and disabling the ingress queue hook

Replace an overcomplicated switch statement with a simple if statement.

This also removes the ingress queue enable outside of nf_hook_mutex as
the protection provided by the mutex is not necessary and the code is
clearer having both of the static key increments together.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>