files: declare fd_params->pos as off_t

Currently pos has type unsigned long, so its size depends on
architecture. pos is saved as 64-bit value in the image file and it
isn't restored, if it is equal to -1. Due to convertation on 32-bit
platforms -1 is converted into UINT_MAX and we get error on restore.

$ zdtm.sh ns/static/tun
...
(00.398513)      5: Error (files-reg.c:534): Can't restore file pos: Illegal seek
(00.398888)      5: Error (files-reg.c:489): Can't open file /dev/net/tun: Illegal seek
...
id: 0x15 flags: 0x2 pos: 0x000000ffffffff fown: { uid: 0 euid: 0 signum: 0 pid_type: 0 pid: 0 }  name: "/dev/net/tun"

crtools is compiled with _FILE_OFFSET_BITS=64, so off_t is always 64-bit.

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>