md/raid10: check slab-out-of-bounds in md_bitmap_get_counter

In the Linux kernel, the following vulnerability has been resolved:

md/raid10: check slab-out-of-bounds in md_bitmap_get_counter

If we write a large number to md/bitmap_set_bits, md_bitmap_checkpage()

will return -EINVAL because 'page >= bitmap->pages', but the return value

was not checked immediately in md_bitmap_get_counter() in order to set

*blocks value and slab-out-of-bounds occurs.

Move check of 'page >= bitmap->pages' to md_bitmap_get_counter() and

return directly if true.