The BtrBulk class, which was introduced in MySQL 5.7, is by design
the exclusive writer to an index. It is therefore unnecessary to
acquire the dict_index_t::lock in that code.

Holding the dict_index_t::lock would unnecessarily block other threads
(SQL connections and the InnoDB purge threads) from buffering concurrent
modifications to being-created secondary indexes.

This fix is motivated by a change in MySQL 5.7.28:
Bug #29008298 MYSQLD CRASHES ITSELF WHEN CREATING INDEX
mysql/mysql-server@f9fb96c20f9d190f654e7aa2387255bf80fd6e45

PageBulk::init(), PageBulk::latch(): Never acquire m_index->lock.

PageBulk::storeExt(): Remove some pointer indirection, and improve
a debug assertion that seems to prove that some code is redundant.

BtrBulk::pageCommit(): Assert that m_index->lock is not being held.

btr_blob_log_check_t: Do not acquire m_index->lock if
m_op == BTR_STORE_INSERT_BULK. Add UNIV_UNLIKELY hints around
that condition.

btr_store_big_rec_extern_fields(): Allow index->lock not to be held
while op == BTR_STORE_INSERT_BULK. Add UNIV_UNLIKELY hints around
that condition.