mmap: find_vma: remove the WARN_ON_ONCE(!mm) check

Remove the WARN_ON_ONCE(!mm) check as the comment suggested.  Kernel
code calls find_vma only when it is absolutely sure that the mm_struct
arg to it is non-NULL.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: k80c <k80ck80c@gmail.com>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1935,9 +1935,6 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
 {
 	struct vm_area_struct *vma = NULL;
 
-	if (WARN_ON_ONCE(!mm))		/* Remove this in linux-3.6 */
-		return NULL;
-
 	/* Check the cache first. */
 	/* (Cache hit rate is typically around 35%.) */
 	vma = ACCESS_ONCE(mm->mmap_cache);