mm/fremap.c: fix possible oops on error path

The vm_flags introduced in 6d7825b1 ("mm/fremap.c: fix oops on error
path") is supposed to avoid a compiler warning about unitialized
vm_flags without changing the generated code.

However I am concerned that this is going to be very brittle, and fail
with some compiler versions. The failure could be either of:

- compiler could actually load vma->vm_flags before checking for the
  !vma condition, thus reintroducing the oops

- compiler could optimize out the !vma check, since the pointer just got
  dereferenced shortly before (so the compiler knows it can't be NULL!)

I propose reversing this part of the change and initializing vm_flags to 0
just to avoid the bogus uninitialized use warning.
Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/fremap.c

@@ -129,7 +129,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
 	struct vm_area_struct *vma;
 	int err = -EINVAL;
 	int has_write_lock = 0;
-	vm_flags_t vm_flags;
+	vm_flags_t vm_flags = 0;
 
 	if (prot)
 		return err;
@@ -163,8 +163,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
 	 * and that the remapped range is valid and fully within
 	 * the single existing vma.
 	 */
-	vm_flags = vma->vm_flags;
-	if (!vma || !(vm_flags & VM_SHARED))
+	if (!vma || !(vma->vm_flags & VM_SHARED))
 		goto out;
 
 	if (!vma->vm_ops || !vma->vm_ops->remap_pages)