shmem: rework getting file descriptors for shared memory regions (v2)

/proc/PID/map_files are protected by the global CAP_SYS_ADMIN, so we
need to avoid using them to support user namespaces.

We are going to use memfd_create() to get the first file descriptor and
then all others processes will able to open it via /proc/PID/fd/X.

This patch reworks slave processes to not use map_files.

v2: add more comments
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>