mm/mmap.c: mark function as static

Mark function as static in mmap.c because they are not used outside this
file.

This eliminates the following warning in mm/mmap.c:

  mm/mmap.c:407:6: warning: no previous prototype for `validate_mm' [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -405,7 +405,7 @@ static void validate_mm_rb(struct rb_root *root, struct vm_area_struct *ignore)
 	}
 }
 
-void validate_mm(struct mm_struct *mm)
+static void validate_mm(struct mm_struct *mm)
 {
 	int bug = 0;
 	int i = 0;