When view is merged by DT_MERGE_FOR_INSERT it is then skipped from
processing and doesn't update WHERE clause with
vers_setup_conds(). Note that view itself cannot work in
vers_setup_conds() because it doesn't have row_start, row_end
fields. Thus it is required to descend down to material TABLE_LIST
through calls of mysql_derived_prepare() and run vers_setup_conds()
from there. Luckily, all views (views of views, views of views of
views, etc.) are linked in one list through next_global pointer, so we
can skip all views of views and get straight to non-view TABLE_LIST by
checking its merge_underlying_list property for zero value (it is
assigned by DT_MERGE_FOR_INSERT for merged derived tables).

We have to do that only for UPDATE and DELETE. Other DML commands
don't use WHERE clause.

MDEV-21146 Assertion `m_lock_type == 2' in handler::ha_drop_table upon LOAD DATA

LOAD DATA does not use WHERE and the above call of vers_setup_conds()
is not needed. unit->prepare() led to wrongly locked temporary table.