Handle pmap_enter() over an existing 4/2M page in KVA on i386.

The userspace case was already handled by pmap_allocpte(). For kernel
VA, page table page must exist, and demote cannot fail, so we need to
just call pmap_demote_pde(). Also note that due to the machine AS
layout, promotions in the KVA on i386 are highly unlikely, so this
change is mostly for completeness.

Reviewed by: alc, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D8323