row_upd_changes_ord_field_binary(): Do not return TRUE if the update
vector changes a column that is covered by a prefix index, but does
not change the column prefix. Add the row_ext_t parameter for
determining whether the prefixes of externally stored columns match.

dfield_datas_are_binary_equal(): Add the parameter len, for comparing
column prefixes when len > 0.

innodb.test: Add a test case where the patch of Bug #55284 failed
without this fix.

rb:537 approved by Jimmy Yang