CVE-2024-50036: net: do not delay dst_entries_add() in dst_release()

In the Linux kernel, the following vulnerability has been resolved:

net: do not delay dstentriesadd() in dstrelease()

dstentriesadd() uses per-cpu data that might be freed at netns dismantle from ip6routenetexit() calling dstentriesdestroy()

Before ip6routenetexit() can be called, we release all the dsts associated with this netns, via calls to dstrelease(), which waits an rcu grace period before calling dstdestroy()

dstentriesadd() use in dstdestroy() is racy, because dstentriesdestroy() could have been called already.

Decrementing the number of dsts must happen sooner.

Notes:

1) in CONFIGXFRM case, dstdestroy() can call dstreleaseimmediate(child), this might also cause UAF if the child does not have DSTNOCOUNT set. IPSEC maintainers might take a look and see how to address this.

2) There is also discussion about removing this count of dst, which might happen in future kernels.