[CIFS] rename cifs_strndup to cifs_strndup_from_ucs

In most cases, cifs_strndup is converting from Unicode (UCS2 / UTF-32) to
the configured local code page for the Linux mount (usually UTF8), so
Jeff suggested that to make it more clear that cifs_strndup is doing
a conversion not just memory allocation and copy, rename the function
to including "from_ucs" (ie Unicode)

Signed-off-by: Steve French <sfrench@us.ibm.com>