gtp: fix wrong condition in gtp_genl_dump_pdp()

gtp_genl_dump_pdp() is ->dumpit() callback of GTP module and it is used
to dump pdp contexts. it would be re-executed because of dump packet size.

If dump packet size is too big, it saves current dump pointer
(gtp interface pointer, bucket, TID value) then it restarts dump from
last pointer.
Current GTP code allows adding zero TID pdp context but dump code
ignores zero TID value. So, last dump pointer will not be found.

In addition, this patch adds missing rcu_read_lock() in
gtp_genl_dump_pdp().

Fixes: 459aa660

 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>