Now we make __mctp_dev_get() take the hold itself. It is safe to test
against the zero refcount because __mctp_dev_get() is called holding
rcu_read_lock and mctp_dev uses kfree_rcu().
Reported-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Matt Johnston <matt@codeconstruct.com.au> Signed-off-by: David S. Miller <davem@davemloft.net>