kernel/sys.c: make prctl(PR_SET_MM) generally available

The purpose of this patch is to allow privileged processes to set
their own per-memory memory-region fields:

      start_code, end_code, start_data, end_data, start_brk, brk,
      start_stack, arg_start, arg_end, env_start, env_end.

This functionality is needed by any application or package that needs to
reconstruct Linux processes, that is, to start them in any way other than
by means of an "execve()" from an executable file.  This includes:

1. Restoring processes from a checkpoint-file (by all potential
   user-level checkpointing packages, not only CRIU's).
2. Restarting processes on another node after process migration.
3. Starting duplicated copies of a running process (for reliability
   and high-availablity).
4. Starting a process from an executable format that is not supported
   by Linux, thus requiring a "manual execve" by a user-level utility.
5. Similarly, starting a process from a networked and/or crypted
   executable that, for confidentiality, licensing or other reasons,
   may not be written to the local file-systems.

The code that does that was already included in the Linux kernel by the
CRIU group, in the form of "prctl(PR_SET_MM)", but prior to this was
enclosed within their private "#ifdef CONFIG_CHECKPOINT_RESTORE", which is
normally disabled.  The patch removes those ifdefs.

Signed-off-by: Amnon Shiloh <u3557@miso.sublimeip.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>