mirror: Avoid reading mirrors with failed devices in mirrored log

mirror:  Avoid reading mirrors with failed devices in mirrored log

Commit 9fd7ac7d035f0b2f8dcc3cb19935eb181816bd76 did not handle mirrors
that contained mirrored logs.  This is because the status line of the
mirror does not give an indication of the health of the mirrored log,
as you can see here:
        [root@bp-01 lvm2]# dmsetup status vg-lv vg-lv_mlog
        vg-lv: 0 409600 mirror 2 253:6 253:7 400/400 1 AA 3 disk 253:5 A
        vg-lv_mlog: 0 8192 mirror 2 253:3 253:4 7/8 1 AD 1 core
Thus, the possibility for LVM commands to hang still persists when mirror
have mirrored logs.  I discovered this while performing some testing that
does polling with 'pvs' while doing I/O and killing devices.  The 'pvs'
managed to get between the mirrored log device failure and the attempt
by dmeventd to repair it.  The result was a very nasty block in LVM
commands that is very difficult to remove - even for someone who knows
what is going on.  Thus, it is absolutely essential that the log of a
mirror be recursively checked for mirror devices which may be failed
as well.

Despite what the code comment says in the aforementioned commit...
+ * _mirrored_transient_status().  FIXME: It is unable to handle mirrors
+ * with mirrored logs because it does not have a way to get the status of
+ * the mirror that forms the log, which could be blocked.
... it is possible to get the status of the log because the log device
major/minor is given to us by the status output of the top-level mirror.
We can use that to query the log device for any DM status and see if it
is a mirror that needs to be bypassed.  This patch does just that and is
now able to avoid reading from mirrors that have failed devices in a
mirrored log.