Problem was that we could take page latches on different
order than wat is entitled with SX-lock. To follow the
latching order defined in WL#6326, acquire index->lock X-latch.
This entitles us to acquire page latches in any order for the index.

btr0btr.cc
	Document latch rules before and after MariaDB 10.2.2

sync0rw.cc
	Document latch compatibility rules better.

btr_defragment_merge_pages
	Fix parameter value.

btr_defragment_thread
	Acquire X-lock to dict_index_t::lock before restoring
	cursor position and continuing defragmentation.

ha_innobase::optimize
	Restore defragment feature.

Testing
	Add GIS-index and FT-index to table being defragmented.

	Defragmentation is not done to GIS-indexes and FT auxiliary
	tables.