mm: move enum vm_event_item into a standalone header file

enums are problematic because they cannot be forward-declared:

  akpm2:/home/akpm> cat t.c

  enum foo;

  static inline void bar(enum foo f)
  {
  }
  akpm2:/home/akpm> gcc -c t.c
  t.c:4: error: parameter 1 ('f') has incomplete type

So move the enum's definition into a standalone header file which can be used
wherever its definition is needed.

Cc: Ying Han <yinghan@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>