btr_cur_t::open_leaf(): When we have to reopen the root page in
a different mode, ensure that we will actually acquire a latch upfront,
instead of using RW_NO_LATCH. This prevents a race condition where
the index tree would be split between the time we released the
root page S latch and finally acquired a latch in
mtr->upgrade_buffer_fix(), actually on a non-leaf root page.

This race condition was introduced in
commit 89ec4b53 (MDEV-29603).