selftests/seccomp: Split fork_and_close into clone and close tests

fork_and_close checked three things against one listener: that a
restarted clone3() creates exactly one child, that a restarted close()
releases its descriptor, and that a supervisor error reaches the caller
after a restart. A three-pass loop chose what each pass did for each
variant, and a failure in the first check kept the others from running.

Split it into notification_restart.clone and notification_restart.close,
each with one mediated syscall, one signal, and one expected result per
variant. They share helpers that fork the filtered child, receive its
listener, and interrupt its notification. notification_restart_child()
now uses the same fork and report helpers, and the filter flags are
computed in one place. Drop the supervisor error check, which
failed_receive already covers.

Tests run on ARCH=x86_64 defconfig with CONFIG_PROVE_LOCKING=y,
CONFIG_DEBUG_ATOMIC_SLEEP=y, and CONFIG_DEBUG_LIST=y under QEMU, built
with GCC 16.2.0: seccomp_bpf passes 137/137 with 5 skips, and the 28
notification_restart tests pass in 25 of 25 runs at -smp 2 and -smp 4.
On v7.3-rc2 the restart and both variants fail and the others pass.

Assisted-by: LLM
Signed-off-by: Kees Cook <kees@kernel.org>

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