netfilter: nft_socket: Fix a NULL vs IS_ERR() bug in nft_socket_cgroup_subtree_level()

The cgroup_get_from_path() function never returns NULL, it returns error
pointers.  Update the error handling to match.

Fixes: 7f3287db ("netfilter: nft_socket: make cgroupsv2 matching work with namespaces")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Link: https://patch.msgid.link/bbc0c4e0-05cc-4f44-8797-2f4b3920a820@stanley.mountain


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