inet: factor out inet_send_prepare()

mainline inclusion
from mainline-v5.3-rc1
commit e4730936
category: bugfix
bugzilla: 187846, https://gitee.com/src-openeuler/kernel/issues/I5W7YP


CVE: CVE-2022-3567

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

The same code is replicated verbatim in multiple places, and the next
patches will introduce an additional user for it. Factor out a
helper and use it where appropriate. No functional change intended.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Xu Jia <xujia39@huawei.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>