PCI: Do not add hotplug reservation multiple times

In nested topologies, hotplug reservations get added multiple times into
the bridge window higher up in the hierarchy. Adding reservations for
intermediate level bridges does not seem very useful because the hotplug is
going to add device at a leaf.

Accounting the hotplug reservation multiple times results in larger than
expected bridge window size that may lead to assignment failures as shown
in this log:

  pci_bus 0000:0a: root bus resource [mem 0x10a00000-0x10c00fff window]
  pci 0000:0a:00.0: BAR 0 [mem 0x10c00000-0x10c00fff]
  pci 0000:0a:00.0:   bridge window [mem 0x10a00000-0x10bfffff]
  pci 0000:0b:00.0:   bridge window [mem 0x10a00000-0x10bfffff]
  pci 0000:0c:02.0:   bridge window [mem 0x10a00000-0x10bfffff]
  pci 0000:0c:02.0: bridge window [mem 0x00100000-0x000fffff 64bit pref] to [bus 0d] add_size 200000 add_align 100000
  pci 0000:0c:02.0: bridge window [mem 0x00100000-0x000fffff] to [bus 0d] add_size 200000 add_align 100000
  pci 0000:0b:00.0: bridge window [mem 0x00100000-0x000fffff 64bit pref] to [bus 0c-0d] add_size 200000 add_align 100000
  pci 0000:0b:00.0: bridge window [mem 0x00100000-0x000fffff] to [bus 0c-0d] add_size 200000 add_align 100000
  pci 0000:0a:00.0: bridge window [mem 0x00100000-0x000fffff 64bit pref] to [bus 0b-0d] add_size 400000 add_align 100000
  pci 0000:0a:00.0: bridge window [mem 0x00100000-0x000fffff] to [bus 0b-0d] add_size 400000 add_align 100000
  pci 0000:0a:00.0: bridge window [mem size 0x00400000]: can't assign; no space
  pci 0000:0a:00.0: bridge window [mem size 0x00400000]: failed to assign
  pci 0000:0a:00.0: bridge window [mem size 0x00400000 64bit pref]: can't assign; no space
  pci 0000:0a:00.0: bridge window [mem size 0x00400000 64bit pref]: failed to assign

The problem stems from calculate_memsize() that calculates size first and
then adds childen size:

  size = max(0, 2M) + 2M

Alter the logic to first account for the children size before applying the
hotplug reservation to not add hotplug reservation multiple times on
different levels of a nested topology. As a result, the size calculation
becomes:

  size = max(0 + 2M, 2M)

Reported-by: Eric Auger <eauger@redhat.com>
Link: https://lore.kernel.org/linux-pci/f23946f2-06ac-4607-8f2c-3ffbc52b627a@redhat.com/
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260720162416.5771-1-ilpo.jarvinen@linux.intel.com