generic/108: tolerate old lvm utility versions

generic/108: tolerate old lvm utility versions

lvm utility in Ubuntu 14.04 LTS treats -l 100%FREE as a hard number
and not as an approximate upper limit.  With ~5G scratch partition
and ~128M scsi_debug device, vg_108 is 1279+31=1310 extents long,
but only 31*2=62 can be allocated with -i 2:

  # lvm lvcreate -i 2 -I 4m -l 100%FREE -n lv_108 vg_108
  Insufficient suitable allocatable extents for logical volume lv_108: 1248 more required

lvm2 commit 4b6e3b5e5ea6 ("allocation: Allow approximate
allocation when specifying size in percent") made '-l 100%FREE'
possible when creating RAID LVs or setting number of stripes.

Fix it by setting the size to allocate to 100M, which is enough for
the test with 128M scsi_debug device.

[eguan: update commit log a bit to mention the lvm2 commit that
changed the lvcreate behavior]

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>