UPSTREAM: wireguard: netlink: avoid variable-sized memcpy on sockaddr

Doing a variable-sized memcpy is slower, and the compiler isn't smart
enough to turn this into a constant-size assignment.

Further, Kees' latest fortified memcpy will actually bark, because the
destination pointer is type sockaddr, not explicitly sockaddr_in or
sockaddr_in6, so it thinks there's an overflow:

    memcpy: detected field-spanning write (size 28) of single field
    "&endpoint.addr" at drivers/net/wireguard/netlink.c:446 (size 16)

Fix this by just assigning by using explicit casts for each checked
case.

Bug: 254441685
Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reported-by:  <syzbot+a448cda4dba2dac50de5@syzkaller.appspotmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit 26c013108c12b94bc023bf19198a4300596c98b1)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I47d7a5d7cf1928d203474e3da5aa47df9df097f1