s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

As demonstrated by commit 74bdf781

 ("genirq: Speedup
show_interrupts()"), irq_desc can be accessed safely in RCU read section.

Hence here resorting to rcu read lock to get rid of irq_lock_sparse().

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Link: https://lore.kernel.org/r/20220422100212.22666-1-kernelfans@gmail.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>