PCI: endpoint: pci-ep-msi: Fix error unwind and prevent double alloc

In the Linux kernel, the following vulnerability has been resolved:

PCI: endpoint: pci-ep-msi: Fix error unwind and prevent double alloc

pci_epf_alloc_doorbell() stores the allocated doorbell message array in epf->db_msg/epf->num_db before requesting MSI vectors. If MSI allocation fails, the array is freed but the EPF state may still point to freed memory.

Clear epf->db_msg and epf->num_db on the MSI allocation failure path so that later cleanup cannot double-free the array and callers can retry allocation.

Also return -EBUSY when doorbells have already been allocated to prevent leaking or overwriting an existing allocation.