dm: split DMF_BLOCK_IO flag into two

Split the DMF_BLOCK_IO flag into two.

DMF_BLOCK_IO_FOR_SUSPEND is set when I/O must be blocked while suspending a
device.  DMF_QUEUE_IO_TO_THREAD is set when I/O must be queued to a
worker thread for later processing.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>