gtp: fix a potential NULL pointer dereference

gtp: fix a potential NULL pointer dereference

When sockfd_lookup() fails, gtp_encap_enable_socket() returns a
NULL pointer, but its callers only check for error pointers thus miss
the NULL pointer case.

Fix it by returning an error pointer with the error code carried from
sockfd_lookup().

(I found this bug during code inspection.)

Fixes: 1e3a3abd8b28 ("gtp: make GTP sockets in gtp_newlink optional")
Cc: Andreas Schultz <aschultz@tpip.net>
Cc: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>
Link: https://patch.msgid.link/20240825191638.146748-1-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>