rxrpc: rxrpc_peer needs to hold a ref on the rxrpc_local record

The rxrpc_peer record needs to hold a reference on the rxrpc_local record
it points as the peer is used as a base to access information in the
rxrpc_local record.

This can cause problems in __rxrpc_put_peer(), where we need the network
namespace pointer, and in rxrpc_send_keepalive(), where we need to access
the UDP socket, leading to symptoms like:

    BUG: KASAN: use-after-free in __rxrpc_put_peer net/rxrpc/peer_object.c:411
    [inline]
    BUG: KASAN: use-after-free in rxrpc_put_peer+0x685/0x6a0
    net/rxrpc/peer_object.c:435
    Read of size 8 at addr ffff888097ec0058 by task syz-executor823/24216

Fix this by taking a ref on the local record for the peer record.

Fixes: ace45bec ("rxrpc: Fix firewall route keepalive")
Fixes: 2baec2c3 ("rxrpc: Support network namespacing")
Reported-by:  <syzbot+b9be979c55f2bea8ed30@syzkaller.appspotmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>