Convert properly UTF-8 to UTF-16

wchar_t is currently 16bit so converting a utf8 encoded characters not
in plane 0 (>= 0x10000) to wchar_t (that is calling char2uni) lead to a
-EINVAL return. This patch detect utf8 in cifs_strtoUTF16 and add special
code calling utf8s_to_utf16s.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>