net: mana: Use pci_name() for debugfs directory naming

net: mana: Use pci_name() for debugfs directory naming

Use pci_name(pdev) for the per-device debugfs directory instead of
hardcoded "0" for PFs and pci_slot_name(pdev->slot) for VFs. The
previous approach had two issues:

1. pci_slot_name() dereferences pdev->slot, which can be NULL for VFs
   in environments like generic VFIO passthrough or nested KVM,
   causing a NULL pointer dereference.

2. Multiple PFs would all use "0", and VFs across different PCI
   domains or buses could share the same slot name, leading to
   -EEXIST errors from debugfs_create_dir().

pci_name(pdev) returns the unique BDF address, is always valid, and is
unique across the system.

Fixes: 6607c17c6c5e ("net: mana: Enable debugfs files for MANA device")
Signed-off-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260408081224.302308-2-ernis@linux.microsoft.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>