Restructure nvlist_nv_alloc to work on FreeBSD

KM_PUSHPAGE is an Illumosism - On FreeBSD it's
aliased to the same malloc flag as KM_SLEEP.
The compiler naturally rejects multiple case
statements with the same value.  This is effectively
a no-op since all callers pass a specific KM_* flag.

Reviewed-by: Jorgen Lundman <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Matt Macy <[email protected]>
Closes openzfs#9643

module/nvpair/nvpair.c

@@ -558,10 +558,10 @@ nvlist_nv_alloc(int kmflag)
 	switch (kmflag) {
 	case KM_SLEEP:
 		return (nv_alloc_sleep);
-	case KM_PUSHPAGE:
-		return (nv_alloc_pushpage);
-	default:
+	case KM_NOSLEEP:
 		return (nv_alloc_nosleep);
+	default:
+		return (nv_alloc_pushpage);
 	}
 #else
 	return (nv_alloc_nosleep);