Fix x86 oprofile regression introduced by:
  commit c34d1b4d
  [PATCH] mm: kill check_user_page_readable

That commit reorganized tests for the userspace stack walking moving all
those tests into dump_backtrace(), however, dump_backtrace() was used for
both userspace and kernel stalk walking.  The result is typically no
recorded callgraph information for kernel samples.

Revive the original function as dump_kernel_backtrace() and rename the
other to dump_user_backtrace() to avoid future confusion.
Signed-off-by: Gerald Britton <gbritton@alum.mit.edu>
Apology-from: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>