exfat: use i_blocksize() to get blocksize

We alreday has the interface i_blocksize() to get blocksize,
so use it.

Signed-off-by: Xianting Tian <tian.xianting@h3c.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Git-commit: 45882a6a0dbd1189defae1a449152f8a8168c274
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: lixiang <lixiang@codeaurora.org>