bcachefs: Fix shutdown ordering

the btree key cache uses the srcu struct created/destroyed by
btree_iter.c; btree_iter needs to be exited last.

Reported-by:  <syzbot+3af9daea347788b15213@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>