Prior to this CL, FreeSpaceForFile() was returning `size_t`, which may
overflow on ILP32 when called on a partition with 4GiB+ free space.
Additionally, it was returning static_cast<size_t>(-1) on error, but the
caller in freecache.cpp didn't check for that.
This CL changes its return type to `int64_t`, and moves the function
into freecache.cpp since there's no external caller.
Test: Run recovery_unit_test on marlin.
Test: Code search shows no external user of FreeSpaceForFile().
Change-Id: I00f501a057726e1f1ab69f367c46c77b30f2d774
49750f15ed