repair: fix an ABBA deadlock in inode prefetching

repair: fix an ABBA deadlock in inode prefetching

The inode prefetching code has a fixed limit of inodes that might are
submitted at a time.  Unfortunately the buffers for them get locked
once the prefetching starts.  That way the threads processing the inode
might get stuck on buffer locked, but not submitted for reading yet.

Fix this by kicking the queue as soon as we would have to wait on the
ra_count semaphore.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reported-by: Arkadiusz Miśkiewicz <arekm@maven.pl>
Tested-by: Arkadiusz Miśkiewicz <arekm@maven.pl>
Signed-off-by: Christoph Hellwig <hch@lst.de>