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

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>
(cherry picked from commit f5beeb1851ea6f8cfcf2657f26cb24c0582b4945)
Signed-off-by: Alex Shi <alex.shi@linaro.org>