Currently, it does a cntlzd on the size and then subtracts it from
21.... this doesn't take into account the varying size of a "long".
Just use __ilog instead (and subtract the 10 we have to subtract
to get to the tsize encoding).

Also correct the comment about page sizes supported.
Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>