The test innodb.innodb-wl5522-debug would occasionally hang
(especially when run with ./mtr --rr) due to a deadlock between
btr_store_big_rec_extern_fields() and dict_stats_analyze_index().
The two threads would acquire the clustered index root page latch and
the tablespace latch in the opposite order. The deadlock was possible
because dict_stats_analyze_index() was holding the index latch in
shared mode and an index root page latch, while waiting for the
tablespace latch. If a stronger dict_index_t::lock had been held
by dict_stats_analyze_index(), any operations that free or allocate
index pages would have been blocked.

In each caller of fseg_n_reserved_pages() except ibuf_init_at_db_start()
which is a special case for ibuf.index at database startup, we must hold
an index latch that prevents concurrent allocation or freeing of index
pages.

Any operation that allocates or free pages that belong to an index tree
must first acquire an index latch in Update or Exclusive mode, and while
holding that, acquire an index root page latch in Update or Exclusive
mode.

dict_index_t::clear(): Also acquire an index latch. Otherwise,
the test innodb.insert_into_empty could hang.

btr_get_size_and_reserved(): Assert that a strong enough index latch
is being held. Only acquire a shared fil_space_t::latch; we are only
reading, not modifying any data.

dict_stats_update_transient_for_index(),
dict_stats_analyze_index(): Acquire a strong enough index latch. Only
acquire a shared fil_space_t::latch.

These operations had followed the same order of acquiring latches in
every InnoDB version since the very beginning
(commit c533308a).
The calls for acquiring tablespace latch had previously been moved in
commit 87839258 and
commit 1e9c922f.

The hang was introduced in
commit 2e814d47 which imported
mysql/mysql-server@ac74632293bea967b352d1b472abedeeaa921b98
which failed to strengthen the locking requirements of the function
btr_get_size().