Modify ipfw's dynamic states KPI.

Hide the locking logic used in the dynamic states implementation from
generic code. Rename ipfw_install_state() and ipfw_lookup_dyn_rule()
function to have similar names: ipfw_dyn_install_state() and
ipfw_dyn_lookup_state(). Move dynamic rule counters updating to the
ipfw_dyn_lookup_state() function. Now this function return NULL when
there is no state and pointer to the parent rule when state is found.
Thus now there is no need to return pointer to dynamic rule, and no need
to hold bucket lock for this state. Remove ipfw_dyn_unlock() function.

Obtained from: Yandex LLC
MFC after: 1 week
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D11657