cifs: fix build_unc_path_to_root to account for a prefixpath

Regression introduced by commit f87d39d9

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Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastryyy@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>