Call pfkeyv2_sysctl_policydumper() with shared netlock. It performs

Call pfkeyv2_sysctl_policydumper() with shared netlock. It performs
read-olny access to netlock protected data, so the radix tree will
not be modified during spd_table_walk() run.

Also change netlock assertion within spd_table_add() and
ipsec_delete_policy() to exclusive. These are correlating functions
which modifies radix tree, so make us sure spd_table_walk() run with
shared netlock is safe.

Feedback and ok by bluhm@