devlink: keep the instance mutex alive until references are gone

The reference needs to keep the instance memory around, but also
the instance lock must remain valid. Users will take the lock,
check registration status and release the lock. mutex_destroy()
etc. belong in the same place as the freeing of the memory.

Unfortunately lockdep_unregister_key() sleeps so we need
to switch the an rcu_work.

Note that the problem is a bit hard to repro, because
devlink_pernet_pre_exit() iterates over registered instances.
AFAIU the instances must get devlink_free()d concurrently with
the namespace getting deleted for the problem to occur.

Reported-by:  <syzbot+d94d214ea473e218fc89@syzkaller.appspotmail.com>
Reported-by:  <syzbot+9f0dd863b87113935acf@syzkaller.appspotmail.com>
Fixes: 9053637e

 ("devlink: remove the registration guarantee of references")
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230111042908.988199-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>