Make fake profile average depth come out right

This isn't perfect as it doesn't really cover all cases. If we need to
generate a profile for some fairly insane cases, the stupid thing just
gives up rather than try to search for a solution that satisfies the
constraints.

For example, if you have a maxdepth=10m and an average depth of 9.9m, it
will try two different (roughly sane) dive profiles, fail to get anything
that matches that kind of dive and that average depth, and then just say
"screw it, I'll match an insane profile instead" and basically generate a
rectangular dive.

It does seem to handle the few cases I tried.

Somebody should double-check my math, and I think we might want to make
the "default profiles" a bit saner. Right now the preferred default
profile simply says "ascent/descent rate of 5m/min, with two levels at
max_depth and 1/3rd maxdepth".

That profile might not work for the simple reason that maybe we did a
quick dive to 40m, but then came up and spent a lot of time in shallower
waters, so our average depth was just 10m or so. Obviously we can *not*
make a dive that has the two depths 40m and 13m come out to an average of
10m deep.

So it has a fallback for things like that, where we try to generate a
profile with a faster ascent/descent rate, and a shallower second depth.

And then there's the fallback with an insane 10m/s ascent/descent rate and
a second depth that is basically at the surface, and if we can't generate
a dive profile with those parameters, either my math is wrong, or somebody
was playing with rockets under-water. So we just give up.

There's a few other heuristics too (if there is no average depth, we
create a random one just to make the profile look reasonable).

I *think* this is all perfectly fine, but I do want others to take a look.
It might be doing something stupid.

[Dirk Hohndel: after the discussion I took the patch and reworded parts of
               the commit message]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

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