btr_free_externally_stored_field(): Pass w=mtr_t::OPT to
note that the BTR_EXTERN_LEN is not necessarily changing
when a multi-page ROW_FORMAT=COMPRESSED off-page column
is being freed, and to allow redundant writes to the redo
log to be optimized away.

Ever since commit 56f6dab1
the refactored function mtr_t::write() asserts by default
that the page contents is being changed.