net/smc: Use sysctl-specified types of buffers in new link group

This patch introduces a new SMC-R specific element buf_type
in struct smc_link_group, for recording the value of sysctl
smcr_buf_type when link group is created.

New created link group will create and reuse buffers of the
type specified by buf_type.

Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>