The function rtr_update_mbr_field_in_place() is generating
MLOG_REC_UPDATE_IN_PLACE or MLOG_COMP_REC_UPDATE_IN_PLACE records
on non-leaf pages, even though MLOG_WRITE_STRING would perfectly
suffice for updating a fixed-length data field.

btr_cur_parse_update_in_place(): If flags==7, the record may be
from rtr_update_mbr_field_in_place(), and we must check if the
page is a leaf page. Otherwise, assume that it is.

btr_cur_update_in_place(): Assert that the page is a leaf page.