datapath: Distribute switch variables for initialization

datapath: Distribute switch variables for initialization

Upstream commit:
    commit 16a556eeb7ed2dc3709fe2c5be76accdfa4901ab
    Author: Kees Cook <keescook@chromium.org>
    Date:   Wed Feb 19 22:23:09 2020 -0800

    openvswitch: Distribute switch variables for initialization

    Variables declared in a switch statement before any case statements
    cannot be automatically initialized with compiler instrumentation (as
    they are not part of any execution flow). With GCC's proposed automatic
    stack variable initialization feature, this triggers a warning (and they
    don't get initialized). Clang's automatic stack variable initialization
    (via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
    doesn't initialize such variables[1]. Note that these warnings (or silent
    skipping) happen before the dead-store elimination optimization phase,
    so even when the automatic initializations are later elided in favor of
    direct initializations, the warnings remain.

    To avoid these problems, move such variables into the "case" where
    they're used or lift them up into the main function body.

    net/openvswitch/flow_netlink.c: In function ‘validate_set’:
    net/openvswitch/flow_netlink.c:2711:29: warning: statement will never be executed [-Wswitch-unreachable]
     2711 |  const struct ovs_key_ipv4 *ipv4_key;
          |                             ^~~~~~~~

    [1] https://bugs.llvm.org/show_bug.cgi?id=44916

    Signed-off-by: Kees Cook <keescook@chromium.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>