[CIFS] Can not mount with prefixpath if root directory of share is inaccessible

Windows allows you to deny access to the top of a share, but permit access to
a directory lower in the path.  With the prefixpath feature of cifs
(ie mounting \\server\share\directory\subdirectory\etc.) this should have
worked if the user specified a prefixpath which put the root of the mount
at a directory to which he had access, but we still were doing a lookup
on the root of the share (null path) when we should have been doing it on
the prefixpath subdirectory.

This fixes Samba bug # 5925

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