bcachefs: Don't put rhashtable on stack

Object debugging generally needs special provisions for putting said
objects on the stack, which rhashtable does not have.

Reported-by:  <syzbot+bcc38a9556d0324c2ec2@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>