mm/fremap.c: fix oops on error path

If find_vma() fails, sys_remap_file_pages() will dereference `vma', which
contains NULL.  Fix it by checking the pointer.

(We could alternatively check for err==0, but this seems more direct)

(The vm_flags change is to squish a bogus used-uninitialised warning
without adding extra code).
Reported-by: Tommi Rantala <tt.rantala@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/fremap.c

@@ -163,7 +163,8 @@ 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.
 	 */
-	if (!vma || !(vma->vm_flags & VM_SHARED))
+	vm_flags = vma->vm_flags;
+	if (!vma || !(vm_flags & VM_SHARED))
 		goto out;
 
 	if (!vma->vm_ops || !vma->vm_ops->remap_pages)
@@ -254,6 +255,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
 	 */
 
 out:
+	if (vma)
 		vm_flags = vma->vm_flags;
 	if (likely(!has_write_lock))
 		up_read(&mm->mmap_sem);