mmap: save some cycles for the shared anonymous mapping

shmem_zero_setup() does not change vm_start, pgoff or vm_flags, only some
drivers change them (such as /driver/video/bfin-t350mcqb-fb.c).

Move these codes to a more proper place to save cycles for shared
anonymous mapping.
Signed-off-by: Huang Shijie <shijie8@gmail.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1191,21 +1191,21 @@ munmap_back:
 			goto unmap_and_free_vma;
 		if (vm_flags & VM_EXECUTABLE)
 			added_exe_file_vma(mm);
+
+		/* Can addr have changed??
+		 *
+		 * Answer: Yes, several device drivers can do it in their
+		 *         f_op->mmap method. -DaveM
+		 */
+		addr = vma->vm_start;
+		pgoff = vma->vm_pgoff;
+		vm_flags = vma->vm_flags;
 	} else if (vm_flags & VM_SHARED) {
 		error = shmem_zero_setup(vma);
 		if (error)
 			goto free_vma;
 	}
 
-	/* Can addr have changed??
-	 *
-	 * Answer: Yes, several device drivers can do it in their
-	 *         f_op->mmap method. -DaveM
-	 */
-	addr = vma->vm_start;
-	pgoff = vma->vm_pgoff;
-	vm_flags = vma->vm_flags;
-
 	if (vma_wants_writenotify(vma))
 		vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);