selftests/seccomp: user_notification_addfd check nextfd is available

selftests/seccomp: user_notification_addfd check nextfd is available

commit 8e3c9f9f3a0742cd12b682a1766674253b33fcf0 upstream.

Currently the user_notification_addfd test checks what the next expected
file descriptor will be by incrementing a variable nextfd. This does not
account for file descriptors that may already be open before the test is
started and will cause the test to fail if any exist.

Replace nextfd++ with a function get_next_fd which will check and return
the next available file descriptor.

Signed-off-by: Terry Tritton <terry.tritton@linaro.org>
Link: https://lore.kernel.org/r/20240124141357.1243457-4-terry.tritton@linaro.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>