RDS: Change send lock from a mutex to a spinlock

This change allows us to call rds_send_xmit() from a tasklet,
which is crucial to our new operating model.

* Change c_send_lock to a spinlock
* Update stats fields "sem_" to "_lock"
* Remove unneeded rds_conn_is_sending()

About locking between shutdown and send -- send checks if the
connection is up. Shutdown puts the connection into
DISCONNECTING. After this, all threads entering send will exit
immediately. However, a thread could be *in* send_xmit(), so
shutdown acquires the c_send_lock to ensure everyone is out
before proceeding with connection shutdown.

Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/stats.c

@@ -57,8 +57,8 @@ static const char *const rds_stat_names[] = {
 	"recv_ping",
 	"send_queue_empty",
 	"send_queue_full",
-	"send_sem_contention",
-	"send_sem_queue_raced",
+	"send_lock_contention",
+	"send_lock_queue_raced",
 	"send_immediate_retry",
 	"send_delayed_retry",
 	"send_drop_acked",