UPSTREAM: fs/proc/kcore.c: Make bounce buffer global for read

Next patch adds bounce buffer for ktext area, so it's
convenient to have single bounce buffer for both
vmalloc/module and ktext cases.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.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: I8f517354e6d12aed75ed4ae6c0a6adef0a1e61da
(cherry picked from commit f5beeb18)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>