bcachefs: Fix a use after free

bcachefs: Fix a use after free

Turns out, we weren't waiting on in flight btree writes when freeing
existing btree nodes. This lead to stray btree writes overwriting newly
allocated buckets, but only started showing itself with some of the
recent allocator work and another patch to move submitting of btree
writes to worqueues.

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