i386: fix handling of Xen entries in final L2 page table

i386: fix handling of Xen entries in final L2 page table

Running Xen on top of KVM exposed an issue that latently also exists
on real hardware: So far, updating any L3 entry resulted in the Xen
owned part of the L2 table referenced by the final L3 one to be re-
initialized. This was not only unnecessary, it actually resulted in
Xen relying on the TLB entry which maps the L2 page that's being
updated not going away intermediately, since as a first step the full
range of Xen owned entries in the L2 were replaced by the respective
ones from the idle page table, and only then the per-domain entries
got re- written to their intended values.

This part of the initialization really is sufficient to be done once,
when the page becomes an L2-with-Xen-entries (PGT_pae_xen_l2) one,
i.e. can be moved to alloc_l2_table(). Only the linear page table
setup has to remain where it always was.

Signed-off-by: Jan Beulich <jbeulich@novell.com>