[PATCH] Remove uninitialized cow if snapshot creation fails

[Jun'ichi Nomura <j-nomura@ce.jp.nec.com>]

Similar to the mirror log cleanup, snapshot cow also should be
removed if the snapshot creation fails.

# dmsetup create --notable testvg-lvol1-cow
# lvcreate -s -l1 -n lvol1 testvg/lvol0
  /dev/testvg/lvol1: not found: device not cleared
  Aborting. Failed to wipe snapshot exception store. Remove new LV and retry.
# lvs
  LV    VG     Attr   LSize Origin Snap%  Move Log Copy% 
  lvol0 testvg -wi-a- 4.00M
  lvol1 testvg -wi--- 4.00M

Attached patch removes the incomplete COW LV if the snapshot
creation fails.

A test script is also attached.

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation of America
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