Do not attempt access beyond the declared end of the dn_blkptr array

This loop in dmu_objset_write_ready():

	for (i = 0; i < dnp->dn_nblkptr; i++)
		bp->blk_fill += dnp->dn_blkptr[i].blk_fill;

invokes _undefined behavior_ for the (common) case of dn_nblkptr=3,
therefore, the compiler is free to do whatever it wants (such as
optimizing it away, or otherwise messing up your expections).

The fix is to be honest about the array size.

Signed-off-by: Tim Chase <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#2511
Closes openzfs#2010

include/sys/dnode.h

@@ -138,9 +138,31 @@ typedef struct dnode_phys {
 
 	uint64_t dn_pad3[4];
 
-	blkptr_t dn_blkptr[1];
-	uint8_t dn_bonus[DN_MAX_BONUSLEN - sizeof (blkptr_t)];
-	blkptr_t dn_spill;
+	/*
+	 * The tail region is 448 bytes, and there are three ways to
+	 * look at it.
+	 *
+	 * 0       64      128     192     256     320     384     448 (offset)
+	 * +---------------+---------------+---------------+-------+
+	 * | dn_blkptr[0]  | dn_blkptr[1]  | dn_blkptr[2]  | /     |
+	 * +---------------+---------------+---------------+-------+
+	 * | dn_blkptr[0]  | dn_bonus[0..319]                      |
+	 * +---------------+-----------------------+---------------+
+	 * | dn_blkptr[0]  | /                     | dn_spill      |
+	 * +---------------+-----------------------+---------------+
+	 */
+	union {
+		blkptr_t dn_blkptr[1+DN_MAX_BONUSLEN/sizeof (blkptr_t)];
+		struct {
+			blkptr_t __dn_ignore1;
+			uint8_t dn_bonus[DN_MAX_BONUSLEN];
+		};
+		struct {
+			blkptr_t __dn_ignore2;
+			uint8_t __dn_ignore3[DN_MAX_BONUSLEN-sizeof (blkptr_t)];
+			blkptr_t dn_spill;
+		};
+	};
 } dnode_phys_t;
 
 typedef struct dnode {