Kernel: Let the user read/write more than one page from/to dev files

Previously reads and writes to /dev/zero, /dev/full, /dev/null and
/dev/random were limited to 4096 bytes.

This removes that restriction so that users can enjoy more zero bytes
in their buffers.

Kernel/Devices/NullDevice.cpp

@@ -43,7 +43,7 @@ KResultOr<size_t> NullDevice::read(FileDescription&, u64, UserOrKernelBuffer&, s
 
 KResultOr<size_t> NullDevice::write(FileDescription&, u64, const UserOrKernelBuffer&, size_t buffer_size)
 {
-    return min(static_cast<size_t>(PAGE_SIZE), buffer_size);
+    return buffer_size;
 }
 
 }