x86/ioapic: Simplify mp_irqdomain_alloc() slightly

x86/ioapic: Simplify mp_irqdomain_alloc() slightly

The IRQ return value of irq_find_mapping() is only tested
for existence, not used for anything else.

So, this call can be replaced by a slightly simpler
irq_resolve_mapping() call, which reduces generated
code size a bit (x86-64 allmodconfig):

   text	   data	    bss	    dec	    hex	filename
  82142	  38633	  18048	 138823	  21e47	arch/x86/kernel/apic/io_apic.o.before
  81932	  38633	  18048	 138613	  21d75	arch/x86/kernel/apic/io_apic.o.after

[ mingo: Fixed & simplified the changelog ]

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-janitors@vger.kernel.org
Link: https://patch.msgid.link/cb3a4968538637aac3a5ae4f5ecc4f5eb43376ea.1760861877.git.christophe.jaillet@wanadoo.fr