Avoid possibly-theoretical OOM crash hazard in hash_create().

One place in hash_create() used DynaHashAlloc() as a convenient
shorthand for MemoryContextAlloc().  That was fine when it was
written, but it stopped being fine when 9c911ec changed
DynaHashAlloc() to use MCXT_ALLOC_NO_OOM (mea culpa).  Change
the code to call plain MemoryContextAlloc() as intended.

I think that this bug may be unreachable in practice, since we now
always create AllocSets with some space already allocated, so that
an OOM failure here for a non-shared hash table should be impossible
(with a hash table name of reasonable length anyway).  And there
aren't enough shared hash tables to make a crash for one of those
probable.  Nonetheless it's clearly not operating as designed, so
back-patch to v16 where 9c911ec came in.

Reported-by: Maksim Korotkov <m.korotkov@postgrespro.ru>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/219bdccd460510efaccf90b57e5e5ef2@postgrespro.ru
Backpatch-through: 16

Author: tglsfdc

src/backend/utils/hash/dynahash.c

@@ -390,7 +390,8 @@ hash_create(const char *tabname, long nelem, const HASHCTL *info, int flags)
 	}
 
 	/* Initialize the hash header, plus a copy of the table name */
-	hashp = (HTAB *) DynaHashAlloc(sizeof(HTAB) + strlen(tabname) + 1);
+	hashp = (HTAB *) MemoryContextAlloc(CurrentDynaHashCxt,
+										sizeof(HTAB) + strlen(tabname) + 1);
 	MemSet(hashp, 0, sizeof(HTAB));
 
 	hashp->tabname = (char *) (hashp + 1);