target: Drop iSCSI use of mutex around max_cmd_sn increment

In a performance profile, taking a mutex in iscsit_increment_maxcmdsn()
shows up very high.  However taking a mutex around "sess->max_cmd_sn += 1"
seems pretty silly: we're not serializing against other contexts in
any useful way.

I did a quick audit and there don't appear to be any other places that
use max_cmd_sn within the mutex more than once, so this lock can't be
providing any useful serialization.

(Get correct values for logging - fix whitespace damage)

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Spencer Baugh <sbaugh@catern.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

include/target/iscsi/iscsi_target_core.h

@@ -637,7 +637,7 @@ struct iscsi_session {
 	/* session wide counter: expected command sequence number */
 	u32			exp_cmd_sn;
 	/* session wide counter: maximum allowed command sequence number */
-	u32			max_cmd_sn;
+	atomic_t		max_cmd_sn;
 	struct list_head	sess_ooo_cmdsn_list;
 
 	/* LIO specific session ID */