net: Fix KABI break for introducing is_skb_wmem()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9HVTH


CVE: CVE-2024-26921

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Fix KABI break when backport commit 7f678def ("skb_expand_head()
adjust skb->truesize incorrectly") by moving is_skb_wmem() into a
private .h file.

Fixes: 7f678def ("skb_expand_head() adjust skb->truesize incorrectly")
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>