Make comment about all-zero pages less scary

The comment about all-zero pages created by smgrzeroextend() sounded
much scarier than the reality. In fact trying to encrypt these
all-zero pages might not only be a waste of CPU cycles but also could
decrease security by making us re-use the same IV first with all zeros
and then with the actual data. And the extra amount of protection
we gain from encrypting them is minuscule since they are only added
at the end of the table, soon overwritten and only gives the attacker
a very slightly more accurate table size.

contrib/pg_tde/src/smgr/pg_tde_smgr.c

@@ -164,24 +164,19 @@ tde_mdreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		BlockNumber bn = blocknum + i;
 		unsigned char iv[16];
 
+		/*
+		 * Detect unencrypted all-zero pages written by smgrzeroextend() by
+		 * looking at the first 32 bytes of the page.
+		 *
+		 * Not encrypting all-zero pages is safe because they are only written
+		 * at the end of the file when extending a table on disk so they tend
+		 * to be short lived plus they only leak a slightly more accurate
+		 * table size than one can glean from just the file size.
+		 */
 		for (int j = 0; j < 32; ++j)
 		{
 			if (((char **) buffers)[i][j] != 0)
 			{
-				/*
-				 * Postgres creates all zero blocks in an optimized route,
-				 * which we do not try
-				 */
-				/* to encrypt. */
-				/*
-				 * Instead we detect if a block is all zero at decryption
-				 * time, and
-				 */
-				/* leave it as is. */
-				/*
-				 * This could be a security issue later, but it is a good
-				 * first prototype
-				 */
 				allZero = false;
 				break;
 			}