gtp: simplify error handling code in 'gtp_encap_enable()'

[ Upstream commit b289ba5e ]

'gtp_encap_disable_sock(sk)' handles the case where sk is NULL, so there
is no need to test it before calling the function.

This saves a few line of code.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 7515e37b ("gtp: allow -1 to be specified as file description from userspace")
Signed-off-by: Sasha Levin <sashal@kernel.org>