ledtrig-cpu: Limit to 8 CPUs

[ Upstream commit abcc131292aa8c7de2c5f0ed76a717436c21de63 ]

Some machines have thousands of CPUs... and trigger mechanisms was not
really meant for thousands of triggers. I doubt anyone uses this
trigger on many-CPU machine; but if they do, they'll need to do it
properly.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Stable-dep-of: ff50f5327613 ("leds: trigger: ledtrig-cpu:: Fix 'output may be truncated' issue for 'cpu'")
Signed-off-by: Sasha Levin <sashal@kernel.org>