kprobes, x86/ptrace.h: Make regs_get_kernel_stack_nth() not fault on bad stack

kprobes, x86/ptrace.h: Make regs_get_kernel_stack_nth() not fault on bad stack

Andy had some concerns about using regs_get_kernel_stack_nth() in a new
function regs_get_kernel_argument() as if there's any error in the stack
code, it could cause a bad memory access. To be on the safe side, call
probe_kernel_read() on the stack address to be extra careful in accessing
the memory. A helper function, regs_get_kernel_stack_nth_addr(), was added
to just return the stack address (or NULL if not on the stack), that will be
used to find the address (and could be used by other functions) and read the
address with kernel_probe_read().

Requested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20181017165951.09119177@gandalf.local.home
Signed-off-by: Ingo Molnar <mingo@kernel.org>