kernel/extable.c: mark core_kernel_text notrace

kernel/extable.c: mark core_kernel_text notrace

core_kernel_text is used by MIPS in its function graph trace processing,
so having this method traced leads to an infinite set of recursive calls
such as:

  Call Trace:
     ftrace_return_to_handler+0x50/0x128
     core_kernel_text+0x10/0x1b8
     prepare_ftrace_return+0x6c/0x114
     ftrace_graph_caller+0x20/0x44
     return_to_handler+0x10/0x30
     return_to_handler+0x0/0x30
     return_to_handler+0x0/0x30
     ftrace_ops_no_ops+0x114/0x1bc
     core_kernel_text+0x10/0x1b8
     core_kernel_text+0x10/0x1b8
     core_kernel_text+0x10/0x1b8
     ftrace_ops_no_ops+0x114/0x1bc
     core_kernel_text+0x10/0x1b8
     prepare_ftrace_return+0x6c/0x114
     ftrace_graph_caller+0x20/0x44
     (...)

Mark the function notrace to avoid it being traced.

Link: http://lkml.kernel.org/r/1498028607-6765-1-git-send-email-marcin.nowakowski@imgtec.com
Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Meyer <thomas@m3y3r.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>