Previously, we were using BTREE_INSERT_RESERVE in a lot of places where
it no longer makes sense.

 - we now have more open_buckets than we used to, and the reserves work
   better, so we shouldn't need to use BTREE_INSERT_RESERVE just because
   we're holding open_buckets pinned anymore.

 - We have the btree key cache for updates to the alloc btree, meaning
   we no longer need the btree reserve to ensure the allocator can make
   forward progress.

This means that we should only need a reserve for btree updates to
ensure that copygc can make forward progress.

Since it's now just for copygc, we can also fold RESERVE_BTREE into
RESERVE_MOVINGGC (the allocator's freelist reserve).
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>