sctp: sysctl: udp_port: avoid using current->nsproxy

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

sctp: sysctl: udp_port: 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, but that would

increase the size of this fix, while 'sctp.ctl_sock' still needs to be

retrieved from 'net' structure.