Backport request for [PATCH 0/3] selftests/seccomp seccomp_bpf test fixes

[Terry Tritton <terry.tritton@xxxxxxxxxx>]

Please backport the following fixes for selftest/seccomp.
I forgot to send the original patches to the stable list!
They resolve some edge cases in the testing.

commit 8e3c9f9f3a0742cd12b682a1766674253b33fcf0
selftests/seccomp: user_notification_addfd check nextfd is available

commit: 471dbc547612adeaa769e48498ef591c6c95a57a
selftests/seccomp: Change the syscall used in KILL_THREAD test

commit: ecaaa55c9fa5e8058445a8b891070b12208cdb6d
selftests/seccomp: Handle EINVAL on unshare(CLONE_NEWPID)

Please backport to 6.6 onwards.

Thanks,
Terry.