9p: Avoid creating multiple slab caches with the same name

In the spirit of [1], avoid creating multiple slab caches with the same
name. Instead, add the dev_name into the mix.

[1]: https://lore.kernel.org/all/20240807090746.2146479-1-pedro.falcato@gmail.com/



Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Reported-by:  <syzbot+3c5d43e97993e1fa612b@syzkaller.appspotmail.com>
Message-ID: <20240807094725.2193423-1-pedro.falcato@gmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>