cifs: add replacement for cifs_strtoUCS_le called cifs_from_ucs2

Add a replacement function for cifs_strtoUCS_le. cifs_from_ucs2
takes args for the source and destination length so that we can ensure
that the function is confined within the intended buffers.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>