xfrm: Fix RCU vs hash_resize_mutex lock inversion

xfrm_bydst_resize() calls synchronize_rcu() while holding
hash_resize_mutex. But then on PREEMPT_RT configurations,
xfrm_policy_lookup_bytype() may acquire that mutex while running in an
RCU read side critical section. This results in a deadlock.

In fact the scope of hash_resize_mutex is way beyond the purpose of
xfrm_policy_lookup_bytype() to just fetch a coherent and stable policy
for a given destination/direction, along with other details.

The lower level net->xfrm.xfrm_policy_lock, which among other things
protects per destination/direction references to policy entries, is
enough to serialize and benefit from priority inheritance against the
write side. As a bonus, it makes it officially a per network namespace
synchronization business where a policy table resize on namespace A
shouldn't block a policy lookup on namespace B.

Fixes: 77cc278f

 (xfrm: policy: Use sequence counters with associated lock)
Cc: stable@vger.kernel.org
Cc: Ahmed S. Darwish <a.darwish@linutronix.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Varad Gautam <varad.gautam@suse.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>