Fix overflow issue when computing ideal size of scratch partition

The type of f_frsize is `unsigned long` which is 32 bit for some system so it will overflow after multiplied by f_bfree. This solution adds one more type casting to ensure the type is 64 bit unsigned integer during the computing.

Bug: 281599020
Test: adb root; adb shell disable-verity
Change-Id: I377ed722d5e245c235c3ae12ff66ac7e91d1d6e8