CVE-2025-21636: sctp: sysctl: plpmtud_probe_interval: avoid using current->nsproxy

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

sctp: sysctl: plpmtudprobeinterval: 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 'net' structure can be obtained from the table->data using containerof().

Note that table->data could also be used directly, as this is the only member needed from the 'net' structure, but that would increase the size of this fix, to use 'data' everywhere 'net->sctp.probeinterval' is used.