lwtunnel: fix error path in lwtunnel_fill_encap()

We recently added a check to see if nla_nest_start() fails.  There are
two issues with that.  First, if it fails then I don't think we should
call nla_nest_cancel().  Second, it's slightly convoluted but the
current code returns success but we should return -EMSGSIZE instead.

Fixes: a50fe0ff ("lwtunnel: check return value of nla_nest_start")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>