rlimit: Properly call security_task_setrlimit

Modify do_prlimit to call security_task_setrlimit passing the task
whose rlimit we are changing not the tsk->group_leader.

In general this should not matter as the lsms implementing
security_task_setrlimit apparmor and selinux both examine the
task->cred to see what should be allowed on the destination task.

That task->cred is shared between tasks created with CLONE_THREAD
unless thread keyrings are in play, in which case both apparmor and
selinux create duplicate security contexts.

So the only time when it will matter which thread is passed to
security_task_setrlimit is if one of the threads of a process performs
an operation that changes only it's credentials.  At which point if a
thread has done that we don't want to hide that information from the
lsms.

So fix the call of security_task_setrlimit.  With the removal
of tsk->group_leader this makes the code slightly faster,
more comprehensible and maintainable.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>