KVM: PPC: Book 3S: XICS: Don't lock twice when checking for resend

This patch improves the code that takes lock twice to check the resend flag
and do the actual resending, by checking the resend flag locklessly, and
add a boolean parameter check_resend to icp_[rm_]deliver_irq(), so the
resend flag can be checked in the lock when doing the delivery.

We need make sure when we clear the ics's bit in the icp's resend_map, we
don't miss the resend flag of the irqs that set the bit. It could be
ordered through the barrier in test_and_clear_bit(), and a newly added
wmb between setting irq's resend flag, and icp's resend_map.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>