Revert "Fix to avoid a compile error due to float to int truncation with GCCGO"

This reverts commit 967a42f116d23051c862a3b4983925de2016f83c.

Signed-off-by: Tatsushi Inagaki <e29253@jp.ibm.com>

Roll back the change to fix the parameter of HumanSize from int64 to float64
Fix to avoid a compilation error of size_test.go with GCCGO due to float to int truncation

Signed-off-by: Tatsushi Inagaki <e29253@jp.ibm.com>