tracing: syscall_regfunc() should not skip kernel threads

syscall_regfunc() ignores the kernel threads because "it has no effect",
see cc3b13c1 "Don't trace kernel thread syscalls" which added this check.

However, this means that a user-space task spawned by call_usermodehelper()
will run without TIF_SYSCALL_TRACEPOINT if sys_tracepoint_refcount != 0.

Remove this check. The unnecessary report from ret_from_fork path mentioned
by cc3b13c1 is no longer possible, see See commit fb45550d76bb5 "make sure
that kernel_thread() callbacks call do_exit() themselves".

A kernel_thread() callback can only return and take the int_ret_from_sys_call
path after do_execve() succeeds, otherwise the kernel will crash. But in this
case it is no longer a kernel thread and thus is needs TIF_SYSCALL_TRACEPOINT.

Link: http://lkml.kernel.org/p/[email protected]

Signed-off-by: Oleg Nesterov <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>

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