Kernel: Send SIGBUS to threads that use after valid Inode mmaped range

According to Dr. POSIX, we should allow to call mmap on inodes even on
ranges that currently don't map to any actual data. Trying to read or
write to those ranges should result in SIGBUS being sent to the thread
that did violating memory access.

Kernel/Memory/PageFaultResponse.h

@@ -10,6 +10,7 @@ namespace Kernel {
 
 enum class PageFaultResponse {
     ShouldCrash,
+    BusError,
     OutOfMemory,
     Continue,
 };