x86/PV: restrict TLB flushing after mod_l[234]_entry()

x86/PV: restrict TLB flushing after mod_l[234]_entry()

Just like we avoid to invoke remote root pt flushes when all uses of an
L4 table can be accounted for locally, the same can be done for all of
L[234] for the linear pt flush when the table is a "free floating" one,
i.e. it is pinned but not hooked up anywhere. While this situation
doesn't occur very often, it can be observed.

Since this breaks one of the implications of the XSA-286 fix, drop the
flush_root_pt_local variable again and set ->root_pgt_changed directly,
just like it was before that change.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>