dm io: remove BIOSET_NEED_RESCUER flag from bios bioset

dm io: remove BIOSET_NEED_RESCUER flag from bios bioset

The BIOSET_NEED_RESCUER flag is only needed when a make_request_fn might
do two allocations from the one bioset, and the second one could block
until the first bio completes.

dm_io() is called from make_request_fn() context.  The closest it comes
to multiple allocations is in chunk_io() in dm-snap-persistent.  But
there the code uses a separate thread to avoid problems.

So BIOSET_NEED_RESCUER is not needed.

Signed-off-by: NeilBrown <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>