PCI: Cache offset of Resizable BAR capability

PCI: Cache offset of Resizable BAR capability

Previously most resizable BAR interfaces (pci_rebar_get_possible_sizes(),
pci_rebar_set_size(), etc) as well as pci_restore_state() searched config
space for a Resizable BAR capability.  Most devices don't have such a
capability, so this is wasted effort, especially for pci_restore_state().

Search for a Resizable BAR capability once at enumeration-time and cache
the offset so we don't have to search every time we need it.  No functional
change intended.

Link: https://lore.kernel.org/r/20250215000301.175097-3-helgaas@kernel.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>