syscall: release a js.Func object in fsCall

syscall: release a js.Func object in fsCall

A js.Func object in fsCall was created for each call but never
released. This CL fixes this.

Change-Id: I2e2b504cbf4fb130b8cfe890a66d3a66aadf56a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/217417
Run-TryBot: Hajime Hoshi <hajimehoshi@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Richard Musiol <neelance@gmail.com>