carl9170: fix debugfs crashes

Ben Greear reported:
> I see lots of instability as soon as I load up the carl9710 NIC.
> My application is going to be poking at it's debugfs files...
>
> BUG: KASAN: slab-out-of-bounds in carl9170_debugfs_read+0xd5/0x2a0
> [carl9170] at addr 0xffff8801bc1208b0
> Read of size 8 by task btserver/5888
> =======================================================================
> BUG kmalloc-256 (Tainted: G W ): kasan: bad access detected
> -----------------------------------------------------------------------
>
> INFO: Allocated in seq_open+0x50/0x100 age=2690 cpu=2 pid=772
>...

This breakage was caused by the introduction of intermediate
fops in debugfs by commit 9fd4dcece43a
("debugfs: prevent access to possibly dead file_operations at file open")

Thankfully, the original/real fops are still available in d_fsdata.

Reported-by: Ben Greear <[e-mail]>
Signed-off-by: Christian Lamparter <[e-mail]>
Cc: stable <[e-mail]> # 4.7+
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/net/wireless/ath/carl9170/debug.c

···
 
 	if (!ar)
 		return -ENODEV;
-	dfops = container_of(file->f_op, struct carl9170_debugfs_fops, fops);
+	dfops = container_of(debugfs_real_fops(file),
+			     struct carl9170_debugfs_fops, fops);
 
 	if (!dfops->read)
 		return -ENOSYS;
···
 
 	if (!ar)
 		return -ENODEV;
-	dfops = container_of(file->f_op, struct carl9170_debugfs_fops, fops);
+	dfops = container_of(debugfs_real_fops(file),
+			     struct carl9170_debugfs_fops, fops);
 
 	if (!dfops->write)
 		return -ENOSYS;