check: run tests in a private pid/mount namespace

check: run tests in a private pid/mount namespace

As mentioned in the previous patch, trying to isolate processes from
separate test instances through the use of distinct Unix process
sessions is annoying due to the many complications with signal handling.

Instead, we could just use nsexec to run the test program with a private
pid namespace so that each test instance can only see its own processes;
and private mount namespace so that tests writing to /tmp cannot clobber
other tests or the stuff running on the main system.  Further, the
process created by the clone(CLONE_NEWPID) call is considered the init
process of that pid namespace, so all processes will be SIGKILL'd when
the init process terminates, so we no longer need systemd scopes for
externally enforced cleanup.

However, it's not guaranteed that a particular kernel has pid and mount
namespaces enabled.  Mount (2.4.19) and pid (2.6.24) namespaces have
been around for a long time, but there's no hard requirement for the
latter to be enabled in the kernel.  Therefore, this bugfix slips
namespace support in alongside the session id thing.

Declaring CONFIG_PID_NS=n a deprecated configuration and removing
support should be a separate conversation, not something that I have to
do in a bug fix to get mainline QA back up.

Note that the new helper cannot unmount the /proc it inherits before
mounting a pidns-specific /proc because generic/504 relies on being able
to read the init_pid_ns (aka systemwide) version of /proc/locks to find
a file lock that was taken and leaked by a process.

Cc: <fstests@vger.kernel.org> # v2024.12.08
Fixes: 8973af00ec212f ("fstests: cleanup fsstress process management")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>