also filed as Bug#13146269, Bug#13713178

btr_get_size(): Add mtr_t parameter. Require that the caller S-latches
index->lock. If index->page==FIL_NULL or the index is to be dropped,
return ULINT_UNDEFINED to indicate that the statistics are
unavailable.

dict_update_statistics(): If btr_get_size() returns ULINT_UNDEFINED,
fake the index cardinality statistics.

dict_index_set_page(): Unused function, remove.

row_drop_table_for_mysql(): Before starting to drop the table, mark
the indexes unavailable in the data dictionary cache while holding
index->lock X-latch.

ha_innobase::prepare_drop_index(), ha_innobase::final_drop_index():
When setting index->to_be_dropped, acquire the index->lock X-latch.

rb:960 approved by Jimmy Yang