[CIFS] Fix paths when share is in DFS to include proper prefix

Some versions of Samba (3.2-pre e.g.) are stricter about checking to make sure that
paths in DFS name spaces are sent in the form \\server\share\dir\subdir ...
instead of \dir\subdir

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