From: CaT <cat@zip.com.au>

What this patch does is allow you to specify the max amount of memory tmpfs
can use as a percentage of available real ram.  This (in my eyes) is useful
so that you do not have to remember to change the setting if you want
something other then 50% and some of your ram goes.

Hugh redid the arithmetic to not overflow at 4GB; the particular order of
lines helps RH's gcc-2.96-110 not to get confused in the do_div.  2.5 can use
totalram_pages.  Update mount options in tmpfs Doc.

There's an argument that the percentage should be of ram+swap, that's what
Christoph originally intended.  But we set the default at 50% of ram only, so
I believe it's more consistent to follow that precedent.