fdinfo: Extract FdinfoEntry from dump_one_file

To support SCMs we'll need to receive them into criu task
(see the SCM patch for details), then dump the received
file as if it was in the dumpee. Then the info about received
descriptor will be written into packet entry.

For this we'll need to perform all the regular file dumping
 code BUT not write the FdinfoEntry into image, so shuffle
the code for that.

The gist of the patch is in two changes -- one in the
do_dump_gen_file(), the other in dump_task_files_seized().
The rest is just tossing the arguments of the functions
relevant to that change.

Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>