mm: /proc/pid/pagemap: hide swap entries from unprivileged users

In commit ab676b7d ("pagemap: do not leak physical addresses to
non-privileged userspace"), the /proc/PID/pagemap is restricted to be
readable only by CAP_SYS_ADMIN to address some security issue.

In commit 1c90308e ("pagemap: hide physical addresses from
non-privileged users"), the restriction is relieved to make
/proc/PID/pagemap readable, but hide the physical addresses for
non-privileged users.

But the swap entries are readable for non-privileged users too.  This
has some security issues.  For example, for page under migrating, the
swap entry has physical address information.  So, in this patch, the
swap entries are hided for non-privileged users too.

Link: http://lkml.kernel.org/r/20180508012745.7238-1-ying.huang@intel.com
Fixes: 1c90308e

 ("pagemap: hide physical addresses from non-privileged users")
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Andrei Vagin <avagin@openvz.org>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Daniel Colascione <dancol@google.com>
Cc: Zi Yan <zi.yan@cs.rutgers.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>