files-reg: discover file system type only if file is going to be dumped

Instead of calling it in fill_fd_params_special (which is called for any found
path).
This reduces amount of system calls on dump.

Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>