vxlan: In vxlan_rcv(), access flags through the vxlan netdevice

vxlan: In vxlan_rcv(), access flags through the vxlan netdevice

vxlan_sock.flags is constructed from vxlan_dev.cfg.flags, as the subset of
flags (named VXLAN_F_RCV_FLAGS) that is important from the point of view of
socket sharing. Attempts to reconfigure these flags during the vxlan netdev
lifetime are also bounced. It is therefore immaterial whether we access the
flags through the vxlan_dev or through the socket.

Convert the socket accesses to netdevice accesses in this separate patch to
make the conversions that take place in the following patches more obvious.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/5d237ffd731055e524d7b7c436de43358d8743d2.1733412063.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>