UPSTREAM: selftests/seccomp: user_notification_addfd check nextfd is available

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.

Change-Id: If8e101d45084b86b130e9678660965809e129aad
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>
(cherry picked from commit 8e3c9f9f)
Signed-off-by: Edward Liaw <edliaw@google.com>