files: Create fake name for remapped file if original name is busy

When live-migrating a container on NFS the link-remap cannot
create original file on target machine, since the file name
is still visible -- source CT is alive. The proper fix would
be to move file aside, then create link, then move original
file back, but since NFS changes file name anyway, we just
do the same -- generate another name for link.

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