net: fix pointer check in skb_pp_cow_data routine

Properly check page pointer returned by page_pool_dev_alloc routine in
skb_pp_cow_data() for non-linear part of the original skb.

Reported-by: Julian Wiedmann <jwiedmann.dev@gmail.com>
Closes: https://lore.kernel.org/netdev/cover.1707729884.git.lorenzo@kernel.org/T/#m7d189b0015a7281ed9221903902490c03ed19a7a


Fixes: e6d5dbdd ("xdp: add multi-buff support for xdp running in generic mode")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Link: https://lore.kernel.org/r/25512af3e09befa9dcb2cf3632bdc45b807cf330.1708167716.git.lorenzo@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>