CIFS: Rename *UCS* functions to *UTF16*

to reflect the unicode encoding used by CIFS protocol.

Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Acked-by: Jeff Layton <jlayton@samba.org>
Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>