[PATCH] dm snapshot: allow zero chunk_size

The chunk size of snapshots cannot be changed so it is redundant to require it
as a parameter when activating an existing snapshot.  Allow a value of zero in
this case and ignore it.  For a new snapshot, use a default value if zero is
specified.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>