gtp: add missing gtp_encap_disable_sock() in gtp_encap_enable()

gtp: add missing gtp_encap_disable_sock() in gtp_encap_enable()

BugLink: https://bugs.launchpad.net/bugs/1839036
[ Upstream commit e30155fd23c9c141cbe7d99b786e10a83a328837 ]

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: 91ed81f9abc7 ("gtp: support SGSN-side tunnels")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>