tpool_dispatch: fail if it cannot start at least 1 worker.
Sponsored by: Axcient
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Alan Somers <asomers@FreeBSD.org>
Closes #16178