netmem: fix skb_frag_address_safe with unreadable skbs

netmem: fix skb_frag_address_safe with unreadable skbs

[ Upstream commit 4672aec56d2e8edabcb74c3e2320301d106a377e ]

skb_frag_address_safe() needs a check that the
skb_frag_page exists check similar to skb_frag_address().

Cc: ap420073@gmail.com
Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250619175239.3039329-1-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>