cifs: fix composing of mount options for DFS referrals

With the change to ignore the unc= and prefixpath= mount options, there
is no longer any need to add them to the options string when mounting.
By the same token, we now need to build a device name that includes the
prefixpath when mounting.

To make things neater, the delimiters on the devicename are changed
to '/' since that's preferred when mounting anyway.

v2: fix some comments and don't bother looking at whether there is
    a prepath in the ref->node_name when deciding whether to pass
    a prepath to cifs_build_devname.

v3: rebase on top of potential buffer overrun fix for stable

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