[ROSE] rose_add_loopback_node: propagate -E

David Binderman's icc logs:
net/rose/rose_route.c(399): remark #593: variable "err" was set but never used

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>