Running test/zdtm.py run -t zdtm/transition/maps008 --snap --pre 30
in a loop I got test failure with "Mem differs"
error each ~20 minutes. The cause of the bug is in lack of
synchronization between processes that still do datagen stage of
test and processes that already do datacheck stage of test.
As a result we can get "Mem differs" error if:
1. Process P1 being on datacheck stage fetches a byte with
   offset X from ashmem vma VMA1 into CPU register.
2. Process P2 still being on datagen stage changes byte with offset X
   in the same ashmem vma VMA1.
3. Process P1 fetches byte with offset X from another ashmem vma VMA2
   mapped to the same physical memory as VMA1.
4. Process P1 compares two fetched bytes and finds out that they
   have different values.
5. Process P1 reports "Mem differs" error.

Race is fixed by waiting for all processes to exit datagen stage
before proceeding to datacheck stage.
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>