Mirror/Thin: Disallow thinpools on mirror logical volumes

Mirror/Thin: Disallow thinpools on mirror logical volumes

The same corner cases that exist for snapshots on mirrors exist for
any logical volume layered on top of mirror.  (One example is when
a mirror image fails and a non-repair LVM command is the first to
detect it via label reading.  In this case, the LVM command will hang
and prevent the necessary LVM repair command from running.)  When
a better alternative exists, it makes no sense to allow a new target
to stack on mirrors as a new feature.  Since, RAID is now capable of
running EX in a cluster and thin is not active-active aware, it makes
sense to pair these two rather than mirror+thinpool.

As further background, here are some additional comments that I made
when addressing a bug related to mirror+thinpool:
(https://bugzilla.redhat.com/show_bug.cgi?id=919604#c9)
I am going to disallow thin* on top of mirror logical volumes.
Users will have to use the "raid1" segment type if they want this.

This bug has come down to a choice between:
1) Disallowing thin-LVs from being used as PVs.
2) Disallowing thinpools on top of mirrors.

The problem is that the code in dev_manager.c:device_is_usable() is unable
to tell whether there is a mirror device lower in the stack from the device
being checked.  Pretty much anything layered on top of a mirror will suffer
from this problem.  (Snapshots are a good example of this; and option #1
above has been chosen to deal with them.  This can also be seen in
dev_manager.c:device_is_usable().)  When a mirror failure occurs, the
kernel blocks all I/O to it.  If there is an LVM command that comes along
to do the repair (or a different operation that requires label reading), it
would normally avoid the mirror when it sees that it is blocked.  However,
if there is a snapshot or a thin-LV that is on a mirror, the above code
will not detect the mirror underneath and will issue label reading I/O.
This causes the command to hang.

Choosing #1 would mean that thin-LVs could never be used as PVs - even if
they are stacked on something other than mirrors.

Choosing #2 means that thinpools can never be placed on mirrors.  This is
probably better than we think, since it is preferred that people use the
"raid1" segment type in the first place.  However, RAID* cannot currently
be used in a cluster volume group - even in EX-only mode.  Thus, a complete
solution for option #2 must include the ability to activate RAID logical
volumes (and perform RAID operations) in a cluster volume group.  I've
already begun working on this.