[CRIU] [PATCH 0/2] mem: Do not re-open files for mappings when not required

[Pavel Emelyanov]

On real apps it's typical to have sequences ov VMAs with
absolutely the same file mapped. We've seen this dump-time
and fixed multiple openings of map_files links with the
file_borrowed flag.

Restore situation is the same -- the vm_open() call in many
cases re-open the same path with the same flags. This slows
things down.

To fix this -- chain VMAs with mapped files to each other
and only the first one opens the file and only the last
one closes it.

Also this thing makes maps06 pass again on setups with low
nrfiles limit.

-- Pavel