net/smc: calculate write offset in RMB only once per connection

Currently, the write offset within the RMB is calculated on each write
operation although it is fixed for each connection. With this patch, the
offset is calculated once and stored in a connection specific variable.

Signed-off-by: Hans Wippel <hwippel@linux.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>