shmem: Rework dumping code to use page-pipe and -xfer engines
shmem: Rework dumping code to use page-pipe and -xfer engines

This makes it easier to merge with anon vmas dumping and makes use of
page server for shared memory.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

tych0/criu,eabatalov/criu,KKoukiou/criu-remote,LK4D4/criu,svloyso/criu,wtf42/criu,sdgdsffdsfff/criu,LK4D4/criu,AuthenticEshkinKot/criu,tych0/criu,rentzsch/criu,eabatalov/criu,kawamuray/criu,fbocharov/criu,LK4D4/criu,fbocharov/criu,kawamuray/criu,efiop/criu,rentzsch/criu,LK4D4/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,tych0/criu,AuthenticEshkinKot/criu,fbocharov/criu,biddyweb/criu,tych0/criu,sdgdsffdsfff/criu,KKoukiou/criu-remote,ldu4/criu,gablg1/criu,marcosnils/criu,marcosnils/criu,ldu4/criu,wtf42/criu,svloyso/criu,biddyweb/criu,ldu4/criu,wtf42/criu,marcosnils/criu,svloyso/criu,svloyso/criu,gonkulator/criu,AuthenticEshkinKot/criu,efiop/criu,biddyweb/criu,LK4D4/criu,gablg1/criu,biddyweb/criu,kawamuray/criu,KKoukiou/criu-remote,AuthenticEshkinKot/criu,ldu4/criu,efiop/criu,gonkulator/criu,fbocharov/criu,tych0/criu,gablg1/criu,gonkulator/criu,gonkulator/criu,eabatalov/criu,rentzsch/criu,gablg1/criu,gablg1/criu,KKoukiou/criu-remote,svloyso/criu,rentzsch/criu,marcosnils/criu,wtf42/criu,gonkulator/criu,gablg1/criu,wtf42/criu,eabatalov/criu,marcosnils/criu,rentzsch/criu,fbocharov/criu,eabatalov/criu,sdgdsffdsfff/criu,KKoukiou/criu-remote,LK4D4/criu,rentzsch/criu,efiop/criu,sdgdsffdsfff/criu,fbocharov/criu,kawamuray/criu,ldu4/criu,ldu4/criu,efiop/criu,efiop/criu,kawamuray/criu,tych0/criu,gonkulator/criu,sdgdsffdsfff/criu,eabatalov/criu,marcosnils/criu,biddyweb/criu,kawamuray/criu,wtf42/criu,AuthenticEshkinKot/criu,sdgdsffdsfff/criu,biddyweb/criu,svloyso/criu