syscall: correct comment in testGetdirentries

syscall: correct comment in testGetdirentries

Correct comment about allocating big enough slice to copy result of
Getdirentries.

While at it, also convert from Dirent directly to slice of byte.

Updates #35092

Change-Id: I892de7953120622882e1561728e1e56b009a2351
Reviewed-on: https://go-review.googlesource.com/c/go/+/202880
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>