Trap cases where the percentage calculation currently leads to an empty
LV and the message:
Internal error: Unable to create new logical volume with no extents
Additionally convert the calculated number of extents from physical to
logical when creating a mirror using a percentage that is based on
Physical Extents. Otherwise a command like 'lvcreate -m3 -l80%FREE'
can never leave any free space.
This brings the behaviour closer to that of lvresize.
(A further patch is needed to cover all the raid types.)