clear_refs: sanitize accepted commands declaration

This is the implementation of the soft-dirty bit concept that should
help keep track of changes in user memory, which in turn is very-very
required by the checkpoint-restore project (http://criu.org).

To create a dump of an application(s) we save all the information about
it to files, and the biggest part of such dump is the contents of tasks'
memory.  However, there are usage scenarios where it's not required to
get _all_ the task memory while creating a dump.  For example, when
doing periodical dumps, it's only required to take full memory dump only
at the first step and then take incremental changes of memory.  Another
example is live migration.  We copy all the memory to the destination
node without stopping all tasks, then stop them, check for what pages
has changed, dump it and the rest of the state, then copy it to the
destination node.  This decreases freeze time significantly.

That said, some help from kernel to watch how processes modify the
contents of their memory is required.

The proposal is to track changes with the help of new soft-dirty bit
this way:

1. First do "echo 4 > /proc/$pid/clear_refs".
   At that point kernel clears the soft dirty _and_ the writable bits from all
   ptes of process $pid. From now on every write to any page will result in #pf
   and the subsequent call to pte_mkdirty/pmd_mkdirty, which in turn will set
   the soft dirty flag.

2. Then read the /proc/$pid/pagemap2 and check the soft-dirty bit reported there
   (the 55'th one). If set, the respective pte was written to since last call
   to clear refs.

The soft-dirty bit is the _PAGE_BIT_HIDDEN one.  Although it's used by
kmemcheck, the latter one marks kernel pages with it, while the former
bit is put on user pages so they do not conflict to each other.

This patch:

A new clear-refs type will be added in the next patch, so prepare
code for that.

[akpm@linux-foundation.org: don't assume that sizeof(enum clear_refs_types) == sizeof(int)]
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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