gtp: allow -1 to be specified as file description from userspace

gtp: allow -1 to be specified as file description from userspace

Existing user space applications maintained by the Osmocom project are
breaking since a recent fix that addresses incorrect error checking.

Restore operation for user space programs that specify -1 as file
descriptor to skip GTPv0 or GTPv1 only sockets.

Fixes: defd8b3c37b0 ("gtp: fix a potential NULL pointer dereference")
Reported-by: Pau Espin Pedrol <pespin@sysmocom.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Tested-by: Oliver Smith <osmith@sysmocom.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241022144825.66740-1-pablo@netfilter.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>