CVE-2025-21635: NULL Pointer Dereference

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

rds: sysctl: rds_tcp_{rcv,snd}buf: avoid using current->nsproxy

As mentioned in a previous commit of this series, using the 'net' structure via 'current' is not recommended for different reasons:

- Inconsistency: getting info from the reader's/writer's netns vs only from the opener's netns.

- current->nsproxy can be NULL in some cases, resulting in an 'Oops' (null-ptr-deref), e.g. when the current task is exiting, as spotted by syzbot [1] using acct(2).

The per-netns structure can be obtained from the table->data using container_of(), then the 'net' one can be retrieved from the listen socket (if available).