autofs4: deal with autofs4_write/autofs4_write races

Just serialize the actual writing of packets into pipe on
a new mutex, independent from everything else in the locking
hierarchy.  As soon as something has started feeding a piece
of packet into the pipe to daemon, we *want* everything else
about to try the same to wait until we are done.

Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/autofs4/autofs_i.h

@@ -116,6 +116,7 @@ struct autofs_sb_info {
 	int needs_reghost;
 	struct super_block *sb;
 	struct mutex wq_mutex;
+	struct mutex pipe_mutex;
 	spinlock_t fs_lock;
 	struct autofs_wait_queue *queues; /* Wait queue pointer */
 	spinlock_t lookup_lock;