selinux: initialize proto variable in selinux_ip_postroute_compat()

stable inclusion
from stable-v5.10.90
commit 4833ad4908a1dff196946b4fd96cb5fe74a8621f
bugzilla: 186168 https://gitee.com/openeuler/kernel/issues/I4SHY1

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4833ad4908a1dff196946b4fd96cb5fe74a8621f



--------------------------------

commit 732bc2ff upstream.

Clang static analysis reports this warning

hooks.c:5765:6: warning: 4th function call argument is an uninitialized
                value
        if (selinux_xfrm_postroute_last(sksec->sid, skb, &ad, proto))
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

selinux_parse_skb() can return ok without setting proto.  The later call
to selinux_xfrm_postroute_last() does an early check of proto and can
return ok if the garbage proto value matches.  So initialize proto.

Cc: stable@vger.kernel.org
Fixes: eef9b416 ("selinux: cleanup selinux_xfrm_sock_rcv_skb() and selinux_xfrm_postroute_last()")
Signed-off-by: Tom Rix <trix@redhat.com>
[PM: typo/spelling and checkpatch.pl description fixes]
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>