rds: don't let RDS shutdown a connection while senders are present

This is the first in a long line of patches that tries to fix races
between RDS connection shutdown and RDS traffic.

Here we are maintaining a count of active senders to make sure
the connection doesn't go away while they are using it.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

net/rds/rds.h

@@ -93,6 +93,7 @@ struct rds_connection {
 
 	spinlock_t		c_send_lock;	/* protect send ring */
 	atomic_t		c_send_generation;
+	atomic_t		c_senders;
 	struct rds_message	*c_xmit_rm;
 	unsigned long		c_xmit_sg;
 	unsigned int		c_xmit_hdr_off;