bcachefs: Fix an allocator shutdown deadlock

bcachefs: Fix an allocator shutdown deadlock

On fstest generic/388, we were seeing sporadic deadlocks in the
emergency shutdown, where we'd get stuck shutting down the allocator
because bch2_btree_update_start() -> bch2_btree_reserve_get() allocated
and then deallocated some btree nodes, putting them back on the
btree_reserve_cache, after the allocator shutdown code had already
cleared out that cache.

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