Ensure that iter->pos always lies between the start and end of iter->k
(the last key returned). Also, bch2_btree_iter_set_pos() now invalidates
the key that peek() or next() returned.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>