exfat: file: fix build with kernel < 4.4.72

`i_blocksize` did not exist on kernel 4.4 until 4.4.72 was released [1].

1. https://lwn.net/Articles/725371/



Signed-off-by: Tianling Shen <i@cnsztl.eu.org>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>