x86/PV: _PAGE_RW changes may take fast path of mod_l[234]_entry()

x86/PV: _PAGE_RW changes may take fast path of mod_l[234]_entry()

The only time _PAGE_RW matters when validating an L2 or higher entry is
when a linear page table is tried to be installed (see the comment ahead
of define_get_linear_pagetable()). Therefore when we disallow such at
build time, we can allow _PAGE_RW changes to take the fast paths there.

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