gtp: add missing gtp_encap_disable_sock() in gtp_encap_enable()

If an invalid role is sent from user space, gtp_encap_enable() will fail.
Then, it should call gtp_encap_disable_sock() but current code doesn't.
It makes memory leak.

Fixes: 91ed81f9 ("gtp: support SGSN-side tunnels")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>