gtp: check skb_pull_data() return in gtp1u_send_echo_resp()

gtp: check skb_pull_data() return in gtp1u_send_echo_resp()

gtp1u_send_echo_resp() ignores skb_pull_data()'s return value. Its
caller gtp1u_udp_encap_recv() only guarantees 16 bytes (udphdr +
gtp1_header), but the pull requests 20 (gtp1_header_long + udphdr). For
a 16-19 byte echo request the pull fails and returns NULL without
advancing skb->data; execution continues, and the following skb_push()
plus the IP header pushed by iptunnel_xmit() move skb->data below
skb->head, tripping skb_under_panic().

Fix it by dropping the packet when skb_pull_data() fails.

  skbuff: skb_under_panic: ...
  kernel BUG at net/core/skbuff.c:214!
  Call Trace:
   skb_push (net/core/skbuff.c:2648)
   iptunnel_xmit (net/ipv4/ip_tunnel_core.c:82)
   gtp_encap_recv (drivers/net/gtp.c:701 drivers/net/gtp.c:808 drivers/net/gtp.c:920)
   udp_queue_rcv_one_skb (net/ipv4/udp.c:2388)
   ...
  Kernel panic - not syncing: Fatal exception in interrupt

Fixes: 9af41cc33471 ("gtp: Implement GTP echo response")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Link: https://patch.msgid.link/20260710230724.942574-1-xmei5@asu.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>