When we started stashing the key being overwritten in
btree_insert_entry, this introduced a typical iterator invalidation
problem, triggered by btree node splits or resorts.

Previously, dealt with this by unconditionally re-validating those
stashed pointers in the transaction commit path. This patch gets rid of
that by doing it only when needed, in bch2_trans_node_add() or
bch2_trans_node_reinit_iter().
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>