With the btree key cache code, we don't need to update the alloc btree
lazily - and this will mean we can remove the bch2_alloc_write() call in
the shutdown path.

Future work: we really need to expend the bucket IO clocks from 16 to 64
bits, so that we don't have to rescale them.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>