ksmbd: fix incorrect AllocationSize set in smb2_get_info

If filesystem support sparse file, ksmbd should return allocated size
using ->i_blocks instead of stat->size. This fix generic/694 xfstests.

Cc: stable@vger.kernel.org
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>