LU-12503 vvp_dev: increment *pos in .next

LU-12503 vvp_dev: increment *pos in .next

As described in

Commit ec2e9995e4c5 ("lustre: llite: change how "dump_page_cache" walks a hash table")

The .next function should increment *pos. For some reason it
didn't, and this can trigger the warning in that function.

Change-Id: If4ac748f455750d82712299b7915eb541a3ddc7e
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35765
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>