net/smc: Scan from current RMB list when no position specified

When finding the first RMB of link group, it should start from the
current RMB list whose index is 0. So fix it.

Fixes: b4ba4652 ("net/smc: extend LLC layer for SMC-Rv2")
Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>