gtp: fix an use-before-init in gtp_newlink()

gtp: fix an use-before-init in gtp_newlink()

*_pdp_find() from gtp_encap_recv() would trigger a crash when a peer
sends GTP packets while creating new GTP device.

RIP: 0010:gtp1_pdp_find.isra.0+0x68/0x90 [gtp]
<SNIP>
Call Trace:
 <IRQ>
 gtp_encap_recv+0xc2/0x2e0 [gtp]
 ? gtp1_pdp_find.isra.0+0x90/0x90 [gtp]
 udp_queue_rcv_one_skb+0x1fe/0x530
 udp_queue_rcv_skb+0x40/0x1b0
 udp_unicast_rcv_skb.isra.0+0x78/0x90
 __udp4_lib_rcv+0x5af/0xc70
 udp_rcv+0x1a/0x20
 ip_protocol_deliver_rcu+0xc5/0x1b0
 ip_local_deliver_finish+0x48/0x50
 ip_local_deliver+0xe5/0xf0
 ? ip_protocol_deliver_rcu+0x1b0/0x1b0

gtp_encap_enable() should be called after gtp_hastable_new() otherwise
*_pdp_find() will access the uninitialized hash table.

Fixes: c470db6d2466 ("gtp: make GTP sockets in gtp_newlink optional")
Signed-off-by: Masahiro Fujiwara <fujiwara.masahiro@gmail.com>
Link: https://lore.kernel.org/r/20201027114846.3924-1-fujiwara.masahiro@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>