repair: use recursive buffer locking

repair: use recursive buffer locking

On a sufficiently corrupt filesystem walking the btree nodes might hit the
same node node again, which currently will deadlock.  Use a recursion
counter to avoid the direct deadlock and let them normal loop detection
(two bad nodes and out) do its work.  This is how repair behaved before
we added the lock when implementing buffer prefetching.

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>