fs: Fix #50619 (again) and add a regression test

fs: Fix #50619 (again) and add a regression test

Bug #50619 was fixed by adding an end_of_stream flag in #50630.
Unfortunately, that fix only applied to the readdir_r() path.  When I
switched Linux to use readdir() in #92778, I inadvertently reintroduced
the bug on that platform.  Other platforms that had always used
readdir() were presumably never fixed.

This patch enables end_of_stream for all platforms, and adds a
Linux-specific regression test that should hopefully prevent the bug
from being reintroduced again.