openwrt: allocator: Fix recommitted pages not being zeroed when madvise is not available.

Support kernels built without CONFIG_ADVISE_SYSCALLS on small
embedded devices.

src/base/allocator/partition_allocator/src/partition_alloc/page_allocator.h

@@ -271,6 +271,8 @@ constexpr PA_COMPONENT_EXPORT(
     PARTITION_ALLOC) bool DecommittedMemoryIsAlwaysZeroed() {
 #if PA_BUILDFLAG(IS_APPLE)
   return false;
+#elif defined(NO_MADVISE_SYSCALL)
+  return false;
 #else
   return true;
 #endif

src/base/allocator/partition_allocator/src/partition_alloc/page_allocator_internals_posix.h

@@ -419,6 +419,9 @@ void DiscardSystemPagesInternal(uintptr_t address, size_t length) {
     ret = madvise(ptr, length, MADV_DONTNEED);
   }
   PA_PCHECK(ret == 0);
+#elif defined(NO_MADVISE_SYSCALL)
+  static_cast<void>(ptr);
+  static_cast<void>(length);
 #else   // PA_BUILDFLAG(IS_APPLE)
   // We have experimented with other flags, but with suboptimal results.
   //