os/bluestore: correct zoned freelist when device write pointers are ahead

os/bluestore: correct zoned freelist when device write pointers are ahead

If we find that the device's write pointers are ahead of ours, then we
need to advance the allocator *and* update the ondisk freelist so that we
don't see the adjustment on the next bluestore restart too.

(Actually, we could perhaps not manage the freelist at all and rely
entirely on the device's write pointers and a metadata scan on startup
w/ the null freelist mode, but it is cheap to track, and I like having
some redundancy so that we see if/when the device does something
unexpected.)

Signed-off-by: Sage Weil <sage@newdream.net>