nexthop: Unlink nexthop group entry in error path

[ Upstream commit 7b01e53e ]

In case of error, remove the nexthop group entry from the list to which
it was previously added.

Fixes: 430a0491 ("nexthop: Add support for nexthop groups")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>