row_ins_clust_index_entry_low(): Invoke btr_set_instant() in the same
mini-transaction that has successfully inserted the metadata record.
In this way, if inserting the metadata record fails before any
undo log record was written for it, the index root page will remain
consistent.

innobase_instant_try(): Remove the btr_set_instant() call.

Reviewed by: Thirunarayanan Balathandayuthapani
Tested by: Matthias Leich