mm/mmap: add inline vma_next() for readability of mmap code

There are three places that the next vma is required which uses the same
block of code.  Replace the block with a function and add comments on what
happens in the case where NULL is encountered.
Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200818154707.2515169-1-Liam.Howlett@Oracle.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -558,6 +558,23 @@ static int find_vma_links(struct mm_struct *mm, unsigned long addr,
 	return 0;
 }
 
+/*
+ * vma_next() - Get the next VMA.
+ * @mm: The mm_struct.
+ * @vma: The current vma.
+ *
+ * If @vma is NULL, return the first vma in the mm.
+ *
+ * Returns: The next VMA after @vma.
+ */
+static inline struct vm_area_struct *vma_next(struct mm_struct *mm,
+					 struct vm_area_struct *vma)
+{
+	if (!vma)
+		return mm->mmap;
+
+	return vma->vm_next;
+}
 static unsigned long count_vma_pages_range(struct mm_struct *mm,
 		unsigned long addr, unsigned long end)
 {
@@ -1128,10 +1145,7 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 	if (vm_flags & VM_SPECIAL)
 		return NULL;
 
-	if (prev)
-		next = prev->vm_next;
-	else
-		next = mm->mmap;
+	next = vma_next(mm, prev);
 	area = next;
 	if (area && area->vm_end == end)		/* cases 6, 7, 8 */
 		next = next->vm_next;
@@ -2632,7 +2646,7 @@ static void unmap_region(struct mm_struct *mm,
 		struct vm_area_struct *vma, struct vm_area_struct *prev,
 		unsigned long start, unsigned long end)
 {
-	struct vm_area_struct *next = prev ? prev->vm_next : mm->mmap;
+	struct vm_area_struct *next = vma_next(mm, prev);
 	struct mmu_gather tlb;
 
 	lru_add_drain();
@@ -2831,7 +2845,7 @@ int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
 		if (error)
 			return error;
 	}
-	vma = prev ? prev->vm_next : mm->mmap;
+	vma = vma_next(mm, prev);
 
 	if (unlikely(uf)) {
 		/*