CIFS: check CIFS_MOUNT_NO_DFS when trying to reuse existing sb

if we mount A then mount A again with nodfs, we shouldn't reuse the
superblock. document the purpose of the defines as well.

there are most likely more flags that needs to be added to this mask,
in fact the logic to find them should be which flag should
be *ignored* when trying to reuse an existing sb.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>