Fix the overflow issue in Checkpoint

When the partition is f2fs and the OS is 32bit, the data.f_bavail and
data.f_frsize are 32 bits in size. The product of them is also 32 bits
in size. If the available size of storage is greater than 4G, the
product may be greater than the unsigned long max value. If the product
is overflow and less than 100M. The UDC feature will be disabled.

There is also an overflow for std::strtoul when the variable content
is a very big number(more the unsigned long max value).

To avoid the overflow:
1. convert the variable data.f_bavvail to uint64_t and then compute the
multiplication.
2. use std::strtoull replace to std::strtoul.

Bug: 147118861
Change-Id: I60172ae4cb7c997e2ad4a36583be74736c25e565