xfrm: annotate data-races around xfrm_policy_count[] and xfrm_policy_default[]

xfrm: annotate data-races around xfrm_policy_count[] and xfrm_policy_default[]

KCSAN reported a data race involving net->xfrm.policy_count access.

Add missing READ_ONCE()/WRITE_ONCE() annotations on
xfrm_policy_count and xfrm_policy_default.

Fixes: 2518c7c2b3d7 ("[XFRM]: Hash policies when non-prefixed.")
Reported-by: syzbot+d85ba1c732720b9a4097@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a2b9e96.99669fcc.12a77b.0006.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>