9p: fix slab cache name creation for real

This was attempted by using the dev_name in the slab cache name, but as
Omar Sandoval pointed out, that can be an arbitrary string, eg something
like "/dev/root". Which in turn trips verify_dirent_name(), which fails
if a filename contains a slash.

So just make it use a sequence counter, and make it an atomic_t to avoid
any possible races or locking issues.

Reported-and-tested-by: Omar Sandoval <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]/
Fixes: 79efebae4afc ("9p: Avoid creating multiple slab caches with the same name")
Acked-by: Vlastimil Babka <[e-mail]>
Cc: Dominique Martinet <[e-mail]>
Cc: Thorsten Leemhuis <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

net/9p/client.c

···
 struct p9_client *p9_client_create(const char *dev_name, char *options)
 {
 	int err;
+	static atomic_t seqno = ATOMIC_INIT(0);
 	struct p9_client *clnt;
 	char *client_id;
 	char *cache_name;
···
 	if (err)
 		goto close_trans;
 
-	cache_name = kasprintf(GFP_KERNEL, "9p-fcall-cache-%s", dev_name);
+	cache_name = kasprintf(GFP_KERNEL,
+		"9p-fcall-cache-%u", atomic_inc_return(&seqno));
 	if (!cache_name) {
 		err = -ENOMEM;
 		goto close_trans;