seq: Avoid some "possible leak" warnings from valgrind.

seq: Avoid some "possible leak" warnings from valgrind.

valgrind regards a block to be "possibly" leaked when no pointers exist to
the beginning of the block but some pointers do point to the middle of the
block.  By moving the hmap_node in struct seq_waiter from the middle of the
struct to the beginning, as this commit does, the pointers to the node from
the hmap in struct seq point to the beginning of the block, which reassures
valgrind.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>