alpha/PCI: Compute legacy size in pci_mmap_legacy_page_range()

alpha/PCI: Compute legacy size in pci_mmap_legacy_page_range()

Currently, pci_mmap_legacy_page_range() reads the legacy resource size from
bus->legacy_mem->size or bus->legacy_io->size.  This couples the mmap
bounds check to the struct pci_bus fields that will be removed when legacy
attributes are converted to static definitions.

Compute the size directly using PCI_LEGACY_MEM_SIZE (0x100000) and
PCI_LEGACY_IO_SIZE (0xffff) macros, and shift by 5 bits for sparse systems.

Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Tested-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Acked-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://patch.msgid.link/20260508043543.217179-21-kwilczynski@kernel.org