PCI/sysfs: Rely on lengths from scnprintf(), dsm_label_utf16s_to_utf8s()

scnprintf() returns the number of bytes written into the buffer.  Change
dsm_label_utf16s_to_utf8s() to do the same.  Rely on those values instead
of using strlen() to compute the buffer length.

No functional change intended.

[bhelgaas: reorder patch in series, len++ to include newline added by
dsm_label_utf16s_to_utf8s(), commit log]
Link: https://lore.kernel.org/r/20210603000112.703037-3-kw@linux.com


Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>