cipso: harden use of skb_cow() in cipso_v4_skbuff_setattr()

cipso: harden use of skb_cow() in cipso_v4_skbuff_setattr()

If skb_cow() is passed a headroom <= -NET_SKB_PAD, it will trigger a
BUG. As a result, use cases should avoid calling with a headroom that
is negative to prevent triggering this issue.

This is the same code pattern fixed in Commit 58fc7342b529 ("ipv6:
BUG() in pskb_expand_head() as part of calipso_skbuff_setattr()").

In cipso_v4_skbuff_setattr(), len_delta can become negative, leading to
a negative headroom passed to skb_cow(). However, the BUG is not
triggerable because the condition headroom <= -NET_SKB_PAD cannot be
satisfied due to limits on the IPv4 options header size.

Avoid potential problems in the future by only using skb_cow() to grow
the skb headroom.

Signed-off-by: Will Rosenberg <whrosenb@asu.edu>
Acked-by: Paul Moore <paul@paul-moore.com>
Link: https://patch.msgid.link/20260120155738.982771-1-whrosenb@asu.edu
Signed-off-by: Paolo Abeni <pabeni@redhat.com>