include/linux/mm.h: provide consistent declaration for num_poisoned_pages

clang reports the following compile warning.

  In file included from mm/vmscan.c:56:
  ./include/linux/swapops.h:327:22: warning:
	section attribute is specified on redeclared variable [-Wsection]
  extern atomic_long_t num_poisoned_pages __read_mostly;
                       ^
  ./include/linux/mm.h:2585:22: note: previous declaration is here
  extern atomic_long_t num_poisoned_pages;
                     ^

Let's use __read_mostly everywhere.

Link: http://lkml.kernel.org/r/1519686565-8224-1-git-send-email-linux@roeck-us.net


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthias Kaehlcke <mka@chromium.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>