mm/mmap.c: extract __vma_unlink_list() as counterpart for __vma_link_list()

Just make the code a little easier to read.

Link: http://lkml.kernel.org/r/20191006012636.31521-3-richardw.yang@linux.intel.com
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/mmap.c b/mm/mmap.c
index 270abd2..148b175 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -686,18 +686,8 @@ static __always_inline void __vma_unlink_common(struct mm_struct *mm,
 						struct vm_area_struct *vma,
 						struct vm_area_struct *ignore)
 {
-	struct vm_area_struct *prev, *next;
-
 	vma_rb_erase_ignore(vma, &mm->mm_rb, ignore);
-	next = vma->vm_next;
-	prev = vma->vm_prev;
-	if (prev)
-		prev->vm_next = next;
-	else
-		mm->mmap = next;
-	if (next)
-		next->vm_prev = prev;
-
+	__vma_unlink_list(mm, vma);
 	/* Kill the cache */
 	vmacache_invalidate(mm);
 }