tracing: Change syscall_*regfunc() to check PF_KTHREAD and use for_each_process_thread()

tracing: Change syscall_*regfunc() to check PF_KTHREAD and use for_each_process_thread()

1. Remove _irqsafe from syscall_regfunc/syscall_unregfunc,
   read_lock(tasklist) doesn't need to disable irqs.

2. Change this code to avoid the deprecated do_each_thread()
   and use for_each_process_thread() (stolen from the patch
   from Frederic).

3. Change syscall_regfunc() to check PF_KTHREAD to skip
   the kernel threads, ->mm != NULL is the common mistake.

   Note: probably this check should be simply removed, needs
   another patch.

[[email protected]: s/do_each_thread/for_each_process_thread/]
Link: http://lkml.kernel.org/p/[email protected]
Signed-off-by: Oleg Nesterov <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>