kernel-boot: Add naming support for sub-function (SF) RDMA devices

PCI Sub-function (SF) is anchored on the auxiliary bus rather than
directly on the PCI bus, so by_pci() previously rejected them as
"Non-PCI" and they fell back to their kernel-assigned name.

Detect this case by treating an "auxiliary" device subsystem as a valid
parent: read the stable 'sfnum' attribute from the aux device and
follow its 'device' symlink up to the underlying PCI BDF. The PCI
parent can be a PF or an SR-IOV VF; feeding the BDF into the
existing get_virtfn_info() / fill_pci_info() path handles both layouts
uniformly. The PCI-derived portion of the name is composed unchanged;
an S<sfnum> suffix is then appended last, after any f<func>/v<vf>
components, since SF identity is independent of them.

Examples:
  SF on a multi-function PF:
    parent 0000:c1:00.0, sfnum 88 -> rocep193s0f0S88
  SF on an SR-IOV VF (VF-SF):
    parent 0000:c1:00.4, sfnum 99 -> rocep193s0f0v0S99

Also update Documentation/udev.md and the rdma-persistent-naming.rules
header comment with the new S<sfnum> suffix.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>