log_free_check(): Assert that the caller must not hold
exclusive lock_sys.latch. This was the case for calls from
ibuf_delete_for_discarded_space(). This caused a deadlock with
another thread that would be holding a latch on a dirty page
that would need to be written so that the checkpoint would advance
and log_free_check() could return. That other thread was waiting
for a shared lock_sys.latch.

fil_delete_tablespace(): Do not invoke ibuf_delete_for_discarded_space()
because in DDL operations, we will be holding exclusive lock_sys.latch.

trx_t::commit(std::vector<pfs_os_file_t>&), innodb_drop_database(),
row_purge_remove_clust_if_poss_low(), row_undo_ins_remove_clust_rec(),
row_discard_tablespace_for_mysql():
Invoke ibuf_delete_for_discarded_space() on the deleted tablespaces after
releasing all latches.