exfat: fix potential wrong error return from get_block

If there is no error, get_block() should return 0. However, when bh_read()
returns 1, get_block() also returns 1 in the same manner.

Let's set err to 0, if there is no error from bh_read()

Fixes: 11a347fb ("exfat: change to get file size from DataLength")
Cc: stable@vger.kernel.org
Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>