Revert "xfrm: destroy xfrm_state synchronously on net exit path"

This reverts commit f75a2804.

With all states (whether user or kern) removed from the hashtables
during deletion, there's no need for synchronous destruction of
states. xfrm6_tunnel states still need to have been destroyed (which
will be the case when its last user is deleted (not destroyed)) so
that xfrm6_tunnel_free_spi removes it from the per-netns hashtable
before the netns is destroyed.

This has the benefit of skipping one synchronize_rcu per state (in
__xfrm_state_destroy(sync=true)) when we exit a netns.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>