MINOR: ebtree: save 8 bytes in struct eb32sc_node

MINOR: ebtree: save 8 bytes in struct eb32sc_node

There is a 4-bytes hole in this structure after the eb_node and the
last field is 4-bytes as well, resulting in a total of 64 bytes with
8 bytes holes. Just moving the key after the eb_node (like in eb32_node)
fills the hole and reduces the structure's size by 8 bytes.