PCI/IOV: Enlarge virtfn sysfs name buffer

The sysfs link name "virtfn%u" constructed by pci_iov_sysfs_link() requires
17 bytes to contain the longest possible string.  Increase VIRTFN_ID_LEN to
accommodate that.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

[bhelgaas: commit log, comment at #define]
Fixes: dd7cc44d ("PCI: add SR-IOV API for Physical Function driver")
Link: https://lore.kernel.org/r/20221218033347.23743-1-gremlin@altlinux.org


Signed-off-by: Alexey V. Vissarionov <gremlin@altlinux.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>