xen/netback: fix build warning

mainline inclusion
from mainline-v6.1
commit 7dfa764e
category: bugfix
bugzilla: 188138
CVE: CVE-2022-3643

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7dfa764e0223a324366a2a1fc056d4d9d4e95491



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Commit ad7f402a ("xen/netback: Ensure protocol headers don't fall in
the non-linear area") introduced a (valid) build warning. There have
even been reports of this problem breaking networking of Xen guests.

Fixes: ad7f402a ("xen/netback: Ensure protocol headers don't fall in the non-linear area")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>