bcachefs: Reserve some open buckets for btree allocations

bcachefs: Reserve some open buckets for btree allocations

This reverts part of the change from "bcachefs: Don't use
BTREE_INSERT_USE_RESERVE so much" - it turns out we still should be
reserving open buckets for btree node allocations, because otherwise
data bucket allocations (especially with erasure coding enabled) can use
up all our open buckets and we won't be able to do the metadata update
that lets us release those open bucket references. Oops.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>