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/arc/kernel/smp.c

@@ -139,7 +139,7 @@ void start_kernel_secondary(void)
 	/* MMU, Caches, Vector Table, Interrupts etc */
 	setup_processor();
 
-	atomic_inc(&mm->mm_users);
+	mmget(mm);
 	mmgrab(mm);
 	current->active_mm = mm;
 	cpumask_set_cpu(cpu, mm_cpumask(mm));