Crossing Dirent.Reclen boundry was manifested in golang/go#28131 as garbaled filenames,
when Dirent.Name was not NULL terminated on FreeBSD due to a bug (parseDirEnt would find a NULL
in the following Dirent's Fileno/Reclen fields).
Only search for NULL on linux, when the Namlen field is available use it directly instead.
Updates golang/go#28131
Change-Id: I64090576c8bad2bd246d1561432bf73d5caee2a9
Reviewed-on: https://go-review.googlesource.com/c/141801
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
