cifs: always do is_path_accessible check in cifs_mount

Currently, we skip doing the is_path_accessible check in cifs_mount if
there is no prefixpath. I have a report of at least one server however
that allows a TREE_CONNECT to a share that has a DFS referral at its
root. The reporter in this case was using a UNC that had no prefixpath,
so the is_path_accessible check was not triggered and the box later hit
a BUG() because we were chasing a DFS referral on the root dentry for
the mount.

This patch fixes this by removing the check for a zero-length
prefixpath.  That should make the is_path_accessible check be done in
this situation and should allow the client to chase the DFS referral at
mount time instead.

Cc: stable@kernel.org
Reported-and-Tested-by: Yogesh Sharma <ysharma@cymer.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>