gtp: fix a potential NULL pointer dereference

In the Linux kernel, the following vulnerability has been resolved: 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.)