mm: rework of dumping shared memory

vma_entry contains shmid and all shared memory are dumped in own files.
The most interesting thing is restore.
A maping is restored by process with the smallest pid. The mamping
is created before executing restorer.
We map a full mapping and restore it's conten, then we open a file from
/proc/pid/map_files and store a descriptor in vma_info. The mapping is
unmaped. Now we can map any region of this mapping in the restorer.

We use this trick, because a target process may have this mapping in
some places and the restorer has not function to open proc files.

v2: fix error hangling
xemul: Fixed static-s and args for cr_dump_shmem

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