x86/entry: Avoid SMAP violation in compat_create_bounce_frame()

x86/entry: Avoid SMAP violation in compat_create_bounce_frame()

A 32bit guest kernel might be running on user mappings.
compat_create_bounce_frame() must whitelist its guest accesses to avoid
risking a SMAP violation.

For both variants of create_bounce_frame(), re-blacklist user accesses if
execution exits via an exception table redirection.

This is XSA-183 / CVE-2016-6259

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>