mm: add new mmget() helper

Apart from adding the helper function itself, the rest of the kernel is
converted mechanically using:

  git grep -l 'atomic_inc.*mm_users' | xargs sed -i 's/atomic_inc(&\(.*\)->mm_users);/mmget\(\1\);/'
  git grep -l 'atomic_inc.*mm_users' | xargs sed -i 's/atomic_inc(&\(.*\)\.mm_users);/mmget\(\&\1\);/'

This is needed for a later patch that hooks into the helper, but might
be a worthwhile cleanup on its own.

(Michal Hocko provided most of the kerneldoc comment.)

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vegard Nossum <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

arch/frv/mm/mmu-context.c

@@ -188,7 +188,7 @@ int cxn_pin_by_pid(pid_t pid)
 		task_lock(tsk);
 		if (tsk->mm) {
 			mm = tsk->mm;
-			atomic_inc(&mm->mm_users);
+			mmget(mm);
 			ret = 0;
 		}
 		task_unlock(tsk);