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

'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>