x86/ioapic.c: simplify add_pin_to_irq_node()

Rather than duplicating the same alloc/init code twice, restructure
the function to look for duplicates and then add an entry
if none is found.

This function is not performance critical; all but one of its callers
are __init functions, and the non-__init caller is for PCI device setup.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>