nscd: Switch to struct scratch_buffer in adhstaiX [BZ #18023]

The pre-allocation of the three scratch buffers increased the initial
stack size somewhat, but if retries are needed, the previous version
used more stack space if extend_alloca could not merge allocations.
Lack of alloca accounting also means could be problematic with
extremely large NSS responses, too.

	[BZ #18023]
	* nscd/aicache.c (addhstaiX): Use struct scratch_buffer instead
	of extend_alloca.