um: time-travel: fix time-travel-start option

[ Upstream commit 7d0a8a49 ]

We need to have the = as part of the option so that the
value can be parsed properly. Also document that it must
be given in nanoseconds, not seconds.

Fixes: 06503870

 ("um: Support time travel mode")
Link: https://patch.msgid.link/20240417102744.14b9a9d4eba0.Ib22e9136513126b2099d932650f55f193120cd97@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>