x86: temporarily disable SMAP to legally access user pages in kernel mode

x86: temporarily disable SMAP to legally access user pages in kernel mode

Use STAC/CLAC to temporarily disable SMAP to allow legal accesses to
user pages in kernel mode

STAC/CLAC is not needed for compat_create_bounce_frame, since in this
chunk of code, it only accesses the pv guest's kernel stack, which is
in ring 1 for 32-bit pv guests.

Signed-off-by: Feng Wu <feng.wu@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>