irqdomain: Add __rcu annotations to radix tree accessors

Fix various address spaces warning of sparse.

kernel/irq/irqdomain.c:1463:14: warning: incorrect type in assignment (different address spaces)
kernel/irq/irqdomain.c:1463:14:    expected void **slot
kernel/irq/irqdomain.c:1463:14:    got void [noderef] <asn:4>**
kernel/irq/irqdomain.c:1465:66: warning: incorrect type in argument 2 (different address spaces)
kernel/irq/irqdomain.c:1465:66:    expected void [noderef] <asn:4>**slot
kernel/irq/irqdomain.c:1465:66:    got void **slot

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: https://lkml.kernel.org/r/1506082841-11530-1-git-send-email-yamada.masahiro@socionext.com