blkcg: change a spin_lock() to spin_lock_irq()

Smatch complains that we re-enable IRQs twice.  It looks like we forgot
to disable them here on the spin_trylock() failure path.  This was added
in 9f13ef678e "blkcg: use double locking instead of RCU for blkg
synchronization".

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>`
Signed-off-by: Jens Axboe <axboe@kernel.dk>