Stop leaking BIOs on rebuild. We used to leak 20k of them on a 20GB
rebuild. Now we leak none.
Also, if g_clone_bp() fails during a rebuild, just stop rebuilding for
a while. Maybe the next step here would be actually reusing the BPs
since we don't really need to clone them for a rebuild (we do for
resync however).
- But we do leak them when we do actual I/O to the disk still :(