repair: avoid ABBA deadlocks on prefetched buffers

repair: avoid ABBA deadlocks on prefetched buffers

Both the prefetch threads and actual repair processing threads can have
multiple buffers at a time locked, but they do no use a common locker
order, which can lead to ABBA deadlocks while trying to lock the buffers.

Switch the prefetch code to do a trylock and skip buffers that have
already been locked to avoid this deadlock.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>