mm/mmap.c: remove useless statement "vma = NULL" in find_vma()

Before the main loop, vma is already is NULL.  There is no need to set it
to NULL again.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Oleg Nesterov <oleg@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

@@ -2047,7 +2047,6 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
 		return vma;
 
 	rb_node = mm->mm_rb.rb_node;
-	vma = NULL;
 
 	while (rb_node) {
 		struct vm_area_struct *tmp;