repair: move rt file block allocation to fill_rtino

repair: move rt file block allocation to fill_rtino

Move the block allocation to fill_rtino to be next to writing to the
files.  That also makes it clear that there is no need to zero the
blocks as we are writing data to all of them.  If repair is interrupted
before finishing to write the blocks it will detect a mismatch on the
next run and regenerate the bitmaps anyway, and a random pattern in
the remaining blocks isn't any worse than zeros.

What really should happen is to write the data to a new file first and
link it in when done, but that's a totally separate project.

Signed-off-by: Christoph Hellwig <hch@lst.de>