cifs: reduce false positives with inode aliasing serverino autodisable

It turns out that not all directory inodes with dentries on the
i_dentry list are unusable here. We only consider them unusable if they
are still hashed or if they have a root dentry attached.

Full disclosure -- this check is inherently racy. There's nothing that
stops someone from slapping a new dentry onto this inode just after
this check, or hashing an existing one that's already attached. So,
this is really a "best effort" thing to work around misbehaving servers.

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