userns: prevent speculative execution

userns: prevent speculative execution

From: Elena Reshetova <elena.reshetova@intel.com>

Since the pos value in function m_start()
seems to be controllable by userspace and later on
conditionally (upon bound check) used to resolve
map->extent, insert an observable speculation
barrier before its usage. This should prevent
observable speculation on that branch and avoid
kernel memory leak.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Orabug: 27340445
CVE: CVE-2017-5753

Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>
Reviewed-by: John Haxby <john.haxby@oracle.com>
Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>