x86/PV: use get_unsafe() instead of copy_from_unsafe()

x86/PV: use get_unsafe() instead of copy_from_unsafe()

The former expands to a single (memory accessing) insn, which the latter
does not guarantee (the __builtin_constant_p() based switch() statement
there is just an optimization). Yet we'd prefer to read consistent PTEs
rather than risking a split read racing with an update done elsewhere.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>