log error instead if disabling IPv6 router advertisement failed
log error instead if disabling IPv6 router advertisement failed

Previously, failing to disable IPv6 router advertisement prevented the daemon to
start.

An issue was reported by a user that started docker using `systemd-nspawn "machine"`,
which produced an error;

    failed to start daemon: Error initializing network controller:
    Error creating default "bridge" network: libnetwork:
    Unable to disable IPv6 router advertisement:
    open /proc/sys/net/ipv6/conf/docker0/accept_ra: read-only file system

This patch changes the error to a log-message instead.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

flavio/docker,Ramzec/docker,Mokolea/docker,moby/moby,jthelin/docker,jimjag/docker,jimjag/docker,gnailuy/docker,trebonian/docker,flavio/docker,flavio/docker,Ramzec/docker,jthelin/docker,mahak/docker,djs55/docker,mahak/docker,yosifkit/docker,jimjag/docker,thaJeztah/docker,moul/docker,gnailuy/docker,paralin/docker,Mokolea/docker,paralin/docker,cpuguy83/docker,Mokolea/docker,moby/moby,konstruktoid/docker-upstream,trebonian/docker,Mokolea/docker,nalind/docker,thaJeztah/docker,bearstech/moby,nawawi/docker,Ramzec/docker,AkihiroSuda/docker,docker/docker,nalind/docker,flavio/docker,cpuguy83/docker,nalind/docker,jameszhan/docker,paralin/docker,paralin/docker,vdemeester/docker,nalind/docker,bearstech/moby,thaJeztah/docker,cpuguy83/docker,konstruktoid/docker-upstream,nawawi/docker,moul/docker,moul/docker,moby/moby,yosifkit/docker,AkihiroSuda/docker,trebonian/docker,Ramzec/docker,moul/docker,konstruktoid/docker-upstream,moby/moby,mahak/docker,gnailuy/docker,bearstech/moby,djs55/docker,nawawi/docker,yosifkit/docker,jimjag/docker,cpuguy83/docker,docker/docker,vdemeester/docker,konstruktoid/docker-upstream,thaJeztah/docker,docker/docker,jameszhan/docker,djs55/docker,AkihiroSuda/docker,jameszhan/docker,mahak/docker,nawawi/docker,bearstech/moby,vdemeester/docker,AkihiroSuda/docker,gnailuy/docker,trebonian/docker,yosifkit/docker