ila: propagate error code in ila_output

This warning:
net/ipv6/ila/ila_lwt.c: In function ‘ila_output’:
net/ipv6/ila/ila_lwt.c:42:6: warning: variable ‘err’ set but not used [-Wunused-but-set-variable]

It looks like the code attempts to set propagate different error
values, but always returned -EINVAL.

Compile tested only. Needs review by original author.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>