net/smc: add comments for smc_link_{usable|sendable}

Add comments for both smc_link_sendable() and smc_link_usable()
to help better distinguish and use them.

No function changes.

Signed-off-by: Dust Li <dust.li@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>