pfsync_update_state() is too paranoid about pf_state::pfsync_state.

pfsync_update_state() is too paranoid about pf_state::pfsync_state.
For example it should not be surprised if caller asks to remove
state from pfsync queue which has been removed already. That kind
of race is sorted out later when pfsync_update_state() calls to
pfsync_q_ins()/pfsync_q_del().  Change relaxes pfsync_update_state()
to panic on sync_state value which is unknown.

OK dlg@