mtr_t::m_freed_pages: Renamed from m_freed_ranges and made it as
pointer indirection.

mtr_t::add_freed_offset(): Allocates m_freed_pages.

mtr_t:clear_freed_ranges(): Removed.

mtr_t::init(): Added debug assertion to check whether m_freed_pages
is not yet initialized.

btr_page_alloc_low(): Remove #ifdef UNIV_DEBUG_SCRUBBING.

mtr_t::commit(): Delete m_freed_pages, reset m_trim_pages and
m_freed_in_system_tablespace.

fil_space_t::clear_freed_ranges(): Added a comment to explain how
undo log tablespaces uses it.