The problem is that dbuf_findbp will return ENOENT if the block it's
trying to find is beyond the end of the file. If that happens, we assume
there is no birth time, and so we lose that information when we write
out new blkptrs. We should teach dbuf_findbp to look for things that are
beyond the current end, but not beyond the absolute end of the file.
To verify, create a large file, truncate it to a short length, and then
write beyond the end. Check with zdb to make sure that there are no
holes with birth time zero (will appear as gaps).