xfs_repair: kick processing thread if ra_count is at limit

xfs_repair: kick processing thread if ra_count is at limit

Zorro hit an xfs_repair hang on a 500T filesystem where
all the prefetch threads were sleeping and nothing progressed.

The problem is that if every buffer we tried to read ahead in
phase6 was already up to date, pf_start_io_workers has no effect;
there is no io to do, and the sem_wait in pf_queuing_worker waits
forever.

Kick the processing thread to avoid this situation.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=201173
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>