BUG/MINOR: haterm: don't apply the default pipe size margin twice

BUG/MINOR: haterm: don't apply the default pipe size margin twice

Commit 6d16b11022 ("BUG/MINOR: haterm: preserve the pipe size margin
for splicing") solved the issue of pipe size being sufficient for the
vmsplice() call, but as Christopher pointed out, the ratio was applied
to the default size of 64k, so now it's applied twice, giving 100k
instead of 80k. Let's drop it from there.

No backport needed.