Remove an obsolete comment that refers to code removed by commit
79c6e4c4. The code and comment related to space consumed by guard
pages in user-space stacks, which we no longer take into account.
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#2722