wireguard: netlink: access device through ctx instead of peer
In the Linux kernel, the following vulnerability has been resolved:

wireguard: netlink: access device through ctx instead of peer

The previous commit fixed a bug that led to a NULL peer->device being
dereferenced. It's actually easier and faster performance-wise to
instead get the device from ctx->wg. This semantically makes more sense
too, since ctx->wg->peer_allowedips.seq is compared with
ctx->allowedips_seq, basing them both in ctx. This also acts as a
defence in depth provision against freed peers.