dm io: simplify dec_count and sync_io

Remove the io struct off the stack in sync_io() and allocate it from
the mempool like is done in async_io().

dec_count() now always calls a callback function and always frees the io
struct back to the mempool (so sync_io and async_io share this pattern).

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>