mm/vmalloc.c: don't dereference possible NULL pointer in __vunmap()

find_vmap_area() can return a NULL pointer and we're going to
dereference it without checking it first. Use the existing
find_vm_area() function which does exactly what we want and checks for
the NULL pointer.

Link: http://lkml.kernel.org/r/[e-mail]
Fixes: f3c01d2f3ade ("mm: vmalloc: avoid racy handling of debugobjects in vunmap")
Signed-off-by: Liviu Dudau <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Cc: Chintan Pandya <[e-mail]>
Cc: Andrey Ryabinin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/vmalloc.c

···
 			addr))
 		return;
 
-	area = find_vmap_area((unsigned long)addr)->vm;
+	area = find_vm_area(addr);
 	if (unlikely(!area)) {
 		WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
 				addr);