This did not work before as we couldn't mark it clean as there would
be some parity blocks out of sync, and raid6 will not assemble a
dirty degraded array.
So make such arrays doubly degraded (the last device becomes a spare)
and clean.

Particularly, add the wiki on osdl.org

Instead of MSW.LSW, just print it as a 64bit number.

array.size is 32bits and counts K.  So for arrays with
more than 4Terrabytes, it can overflow.
The correct number can be read from sysfs, but there are still
a few places that use array.size and risk truncation.  What is worse.
they compare a number of kilobytes with a number of sectors !!

So use get_component_size() to read the sysfs information, and be
more consistent about units.

Some kernel versions don't put a space between 'active' and '(auto-read-only)'
in /proc/mdstat.  This causes a parsing problem leaving 'level' set to
NULL which causes a crash.

So synthesise a space there if it is missing, and check for 'level' to
be NULL and don't de-ref if it is.