/proc/pid/pagemap is protected by the global CAP_SYS_ADMIN, so
it can't be opened in a non-root userns.

00:34:48.260 Execute ns/user/static/cow01
...
00:34:48.260 12:37:03.951:     4: ERR: cow01.c:87: Unable to open child pagemap file /proc/5/pagemap (errno = 1 (Operation not permitted))

Reported-by: Mr Jenkins
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>