UPSTREAM: fs/proc/kcore.c: Add bounce buffer for ktext data

We hit hardened usercopy feature check for kernel text access by reading
kcore file:

  usercopy: kernel memory exposure attempt detected from ffffffff8179a01f (<kernel text>) (4065 bytes)
  kernel BUG at mm/usercopy.c:75!

Bypassing this check for kcore by adding bounce buffer for ktext data.

Reported-by: Steve Best <sbest@redhat.com>
Fixes: f5509cc1 ("mm: Hardened usercopy")
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Bug: 31374226
Change-Id: Ic93e6041b67d804a994518bf4950811f828b406e
(cherry picked from commit df04abfd)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>