Mirrors can be layered - as in the case of an converting 2-way

Mirrors can be layered - as in the case of an converting 2-way
to 3-way mirror.  When conversion operations are performed on
these types of mirrors, log options can be confused/ignored.

In the case of a converting 3-way mirror, we have a top-level
2-way corelog mirror whose legs are 1) a 2-way disk-log mirror
and 2) a linear device.  If we wish to convert this 3-way mirror
to a 2-way mirror, the linear device is removed and the extra
top layer is eliminated.  If we also wished to convert the disk
log to a core log in the same step, ambiguity creeps in.  It is
somewhat obvious what the user wants - a 2-way mirror with a
corelog.  However, looking at the top level mirror before
compression, it seems that the mirror already has a core log.
This is why the operation seemed to fail.

This patch simply re-evaluates what mirrored_seg points to after
a compression and then considers the log argument.

This is a fix for bug 599898.