ovpn: zero-initialize sockaddr before learning a floated endpoint

ovpn: zero-initialize sockaddr before learning a floated endpoint

ovpn_peer_endpoints_update() builds the new remote endpoint in an
on-stack struct sockaddr_storage that is left uninitialized. For IPv4
only sin_family/sin_addr/sin_port are written, leaving the 8-byte
sin_zero padding as stack garbage (for IPv6, sin6_flowinfo is left
uninitialized likewise).

ovpn_peer_reset_sockaddr() -> ovpn_bind_from_sockaddr() then memcpy()s
sizeof(struct sockaddr_in)/sizeof(struct sockaddr_in6) bytes - padding
included - into bind->remote. That buffer is later hashed with jhash()
over the same length to place the peer in the by_transp_addr table, so
the garbage padding lands the floated peer in an essentially random
bucket. Lockless lookups in ovpn_peer_get_by_transp_addr() build their
key from a zero-initialized sockaddr_storage, compute a different bucket
and fail to find the peer.

This is also a plain use of uninitialized stack memory in jhash().

Build the floated endpoint with a designated initializer so the
padding (sin_zero for IPv4, sin6_flowinfo for IPv6) is zeroed as part
of the assignment. This keeps the padding out of the by_transp_addr
hash key without memset-ing the whole sockaddr_storage on every
received packet.

Fixes: f0281c1d3732 ("ovpn: add support for updating local or remote UDP endpoint")
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>