sctp: sysctl: rto_min/max: avoid using current->nsproxy

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

sctp: sysctl: rto_min/max: 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

container_of().

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.rto_min/max' is used.