Fix harmless thinko in dsa.c.

Commit 16be2fd100 added DSA_ALLOC_HUGE,
DSA_ALLOC_ZERO and DSA_ALLOC_NO_OOM which have the same numerical
values and meanings as the similarly named MCXT_... macros.  In one
place we accidentally used MCXT_ALLOC_NO_OOM when DSA_ALLOC_NO_OOM is
wanted, so tidy that up.

Author: Thomas Munro
Discussion: http://postgr.es/m/CAEepm=2AimHxVkkxnMfQvbZMkXy0uKbVa0-D38c5-qwrCm4CMQ@mail.gmail.com
Backpatch: 10, where dsa was introduced.

src/backend/utils/mmgr/dsa.c

@@ -707,7 +707,7 @@ dsa_allocate_extended(dsa_area *area, Size size, int flags)
 			dsa_free(area, span_pointer);
 
 			/* Raise error unless asked not to. */
-			if ((flags & MCXT_ALLOC_NO_OOM) == 0)
+			if ((flags & DSA_ALLOC_NO_OOM) == 0)
 				ereport(ERROR,
 						(errcode(ERRCODE_OUT_OF_MEMORY),
 						 errmsg("out of memory"),