bcachefs: Rework btree iterator lifetimes

bcachefs: Rework btree iterator lifetimes

The btree_trans struct needs to memoize/cache btree iterators, so that
on transaction restart we don't have to completely redo btree lookups,
and so that we can do them all at once in the correct order when the
transaction had to restart to avoid a deadlock.

This switches the btree iterator lookups to work based on iterator
position, instead of trying to match them up based on the stack trace.

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