[CRIU] [PATCH 0/3] page-read: Add async mode

[Pavel Emelyanov]

For lazy pages reading we need the ability to do ->read_pages callback
but not expect that it would return any data in the buffer.

Also, for regular restore, we don't need the data in memory immediately,
but agree to wait.

So here is the async page-read mode. For local case it's not truly async,
as it still calls blocking read() syscall. But in Linux kernel there's
no method for doing asynchronous read from page cache.