Check that THP is not set to always (madvise is ok) (#4001)

THP can also be set to madvise, in which case it shouldn't cause
problems for Redis since redis (or the allocator) doesn't use madvise
to activate it.

(cherry picked from commit b2419c31c1)

src/latency.c

@@ -71,7 +71,7 @@ int THPIsEnabled(void) {
         return 0;
     }
     fclose(fp);
-    return (strstr(buf,"[never]") == NULL) ? 1 : 0;
+    return (strstr(buf,"[always]") != NULL) ? 1 : 0;
 }
 #endif