Add defenses against running with a wrong selection of LOBLKSIZE.

It's critical that the backend's idea of LOBLKSIZE match the way data has
actually been divided up in pg_largeobject.  While we don't provide any
direct way to adjust that value, doing so is a one-line source code change
and various people have expressed interest recently in changing it.  So,
just as with TOAST_MAX_CHUNK_SIZE, it seems prudent to record the value in
pg_control and cross-check that the backend's compiled-in setting matches
the on-disk data.

Also tweak the code in inv_api.c so that fetches from pg_largeobject
explicitly verify that the length of the data field is not more than
LOBLKSIZE.  Formerly we just had Asserts() for that, which is no protection
at all in production builds.  In some of the call sites an overlength data
value would translate directly to a security-relevant stack clobber, so it
seems worth one extra runtime comparison to be sure.

In the back branches, we can't change the contents of pg_control; but we
can still make the extra checks in inv_api.c, which will offer some amount
of protection against running with the wrong value of LOBLKSIZE.

src/include/storage/large_object.h

@@ -66,6 +66,8 @@ typedef struct LargeObjectDesc
  * Also, it seems to be a smart move to make the page size be a power of 2,
  * since clients will often be written to send data in power-of-2 blocks.
  * This avoids unnecessary tuple updates caused by partial-page writes.
+ *
+ * NB: Changing LOBLKSIZE requires an initdb.
  */
 #define LOBLKSIZE		(BLCKSZ / 4)