Compare prepaths when comparing superblocks

The patch
fs/cifs: make share unaccessible at root level mountable
makes use of prepaths when any component of the underlying path is
inaccessible.

When mounting 2 separate shares having different prepaths but are other
wise similar in other respects, we end up sharing superblocks when we
shouldn't be doing so.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Tested-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <smfrench@gmail.com>