smb: Use loff_t for directory position in cached_dirents

Change the pos field in struct cached_dirents from int to loff_t
to support large directory offsets. This avoids overflow and
matches kernel conventions for directory positions.

Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Signed-off-by: Bharath SM <bharathsm@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>