x86/PCI: Use struct_size() in pirq_convert_irt_table()

Use struct_size() instead of hand-writing it. It is less verbose, more
robust and more informative.

Link: https://lore.kernel.org/r/00a5cc2cd322e7dea26579916ac6dda9c637aa57.1684518118.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>