PCI: Avoid reset when disabled via sysfs

After d88f521da3ef ("PCI: Allow userspace to query and set device reset
mechanism"), userspace can disable reset of specific PCI devices by writing
an empty string to the sysfs reset_method file.

However, pci_slot_resettable() does not check pci_reset_supported(), which
means that pci_reset_function() will still reset the device even if
userspace has disabled all the reset methods.

I was able to reproduce this issue with a vfio device passed to a qemu
guest, where I had disabled PCI reset via sysfs.

Add an explicit check of pci_reset_supported() in both
pci_slot_resettable() and pci_bus_resettable() to ensure both the reset
status and reset execution are bypassed if an administrator disables it for
a device.

Link: https://lore.kernel.org/r/20250207205600.1846178-1-naravamudan@nvidia.com
Fixes: d88f521da3ef ("PCI: Allow userspace to query and set device reset mechanism")
Signed-off-by: Nishanth Aravamudan <naravamudan@nvidia.com>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Raphael Norwitz <raphael.norwitz@nutanix.com>
Cc: Amey Narkhede <ameynarkhede03@gmail.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Yishai Hadas <yishaih@nvidia.com>
Cc: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Cc: Kevin Tian <kevin.tian@intel.com>

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