Revert "sctp: sysctl: auth_enable: avoid using current->nsproxy"

This reverts commit 10c869a5 as it
was backported incorrectly.
A subsequent commit will re-backport the original patch.

Signed-off-by: Magali Lemes <magali.lemes@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>