md: only calculate blocksize once and use i_blocksize()

We alreday has the interface i_blocksize(), which can be used
to get blocksize, so use it.
Only calculate blocksize once and use it within read_page().

Signed-off-by: Xianting Tian <tian.xianting@h3c.com>
Signed-off-by: Song Liu <songliubraving@fb.com>