btrfs: don't prematurely free work in scrub_missing_raid56_worker()

Currently, scrub_missing_raid56_worker() puts and potentially frees
sblock (which embeds the work item) and then submits a bio through
scrub_wr_submit(). This is another potential instance of the bug in
"btrfs: don't prematurely free work in run_ordered_work()". Fix it by
dropping the reference after we submit the bio.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>