Image a process has done shmget(2 pages), then shmat() then
mprotect(1 page, ro). In this case criu will dump 1 shmem
segment 2 pages long and 2 vmas 1 page each.

But on restore time we'll call shmat() for _each_ vma and the
very first one will occupy the whole 2 pages space in vm
(there's no size argument for shmat, only for shmget) thus
blocking the 2nd vma from shmat()-in again.

The solution is:

1. check that each shmem segment is attached by
   the sequence of vmas that cover one w/o holes

2. shmat() only the first one

3. mprotect() all of them if needed (there's no hunks
   for this step in this path, mprotect is already
   called in pie/restorer.c and does things well)

v2:

* List can contain anon shmems (caught by zdtm)
* There can be many attachments of a segment (caught by transition/ipc)
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>