x86/mm: Fix up comments around PMD preallocation

The "paravirt environment" is no longer in the tree. Axe that part of the
comment. Also add a blurb to remind readers that "USER_PMDS" refer to
the PTI user *copy* of the page tables, not the user *porttion*.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
1 file changed