dm snapshot: require non zero chunk size by end of ctr

dm snapshot: require non zero chunk size by end of ctr

If we are creating snapshot with memory-stored exception store, fail if
the user didn't specify chunk size. Zero chunk size would probably crash
a lot of places in the rest of snapshot code.

Cc: [email protected]
Signed-off-by: Mikulas Patocka <[email protected]>
Reviewed-by: Jonathan Brassow <[email protected]>
Reviewed-by: Mike Snitzer <[email protected]>
Signed-off-by: Alasdair G Kergon <[email protected]>