Mostly-cosmetic improvements in memory chunk header alignment coding.

Add commentary about what we're doing and why.  Apply the method used for
padding in GenerationChunk to AllocChunkData, replacing the rather ad-hoc
solution used in commit 7e3aa03b4.  Reorder fields in GenerationChunk so
that the padding calculation will work even if sizeof(size_t) is different
from sizeof(void *) --- likely that will never happen, but we don't need
the assumption if we do it like this.  Improve static assertions about
alignment.

In passing, fix a couple of oversights in the "large chunk" path in
GenerationAlloc().

Discussion: https://postgr.es/m/E1eHa4J-0006hI-Q8@gemulon.postgresql.org

src/backend/utils/mmgr/aset.c

@@ -157,6 +157,14 @@ typedef struct AllocBlockData
 /*
  * AllocChunk
  *		The prefix of each piece of memory in an AllocBlock
+ *
+ * Note: to meet the memory context APIs, the payload area of the chunk must
+ * be maxaligned, and the "aset" link must be immediately adjacent to the
+ * payload area (cf. GetMemoryChunkContext).  We simplify matters for this
+ * module by requiring sizeof(AllocChunkData) to be maxaligned, and then
+ * we can ensure things work by adding any required alignment padding before
+ * the "aset" field.  There is a static assertion below that the alignment
+ * is done correctly.
  */
 typedef struct AllocChunkData
 {
@@ -166,15 +174,19 @@ typedef struct AllocChunkData
 	/* when debugging memory usage, also store actual requested size */
 	/* this is zero in a free chunk */
 	Size		requested_size;
-#if MAXIMUM_ALIGNOF > 4 && SIZEOF_VOID_P == 4
-	Size		padding;
-#endif
 
+#define ALLOCCHUNK_RAWSIZE  (SIZEOF_SIZE_T * 2 + SIZEOF_VOID_P)
+#else
+#define ALLOCCHUNK_RAWSIZE  (SIZEOF_SIZE_T + SIZEOF_VOID_P)
 #endif							/* MEMORY_CONTEXT_CHECKING */
 
+	/* ensure proper alignment by adding padding if needed */
+#if (ALLOCCHUNK_RAWSIZE % MAXIMUM_ALIGNOF) != 0
+	char		padding[MAXIMUM_ALIGNOF - ALLOCCHUNK_RAWSIZE % MAXIMUM_ALIGNOF];
+#endif
+
 	/* aset is the owning aset if allocated, or the freelist link if free */
 	void	   *aset;
-
 	/* there must not be any padding to reach a MAXALIGN boundary here! */
 }			AllocChunkData;
 
@@ -327,8 +339,11 @@ AllocSetContextCreate(MemoryContext parent,
 {
 	AllocSet	set;
 
+	/* Assert we padded AllocChunkData properly */
+	StaticAssertStmt(ALLOC_CHUNKHDRSZ == MAXALIGN(ALLOC_CHUNKHDRSZ),
+					 "sizeof(AllocChunkData) is not maxaligned");
 	StaticAssertStmt(offsetof(AllocChunkData, aset) + sizeof(MemoryContext) ==
-					 MAXALIGN(sizeof(AllocChunkData)),
+					 ALLOC_CHUNKHDRSZ,
 					 "padding calculation in AllocChunkData is wrong");
 
 	/*