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.

To implement this restriction, we simply check if the result of
read_bytes on an Inode returns 0, which means we have nothing valid to
map to the program, hence it should receive a SIGBUS in that case.

Kernel/Memory/Region.cpp

@@ -501,6 +501,11 @@ PageFaultResponse Region::handle_inode_fault(size_t page_index_in_region)
     }
 
     auto nread = result.value();
+    // Note: If we received 0, it means we are at the end of file or after it,
+    // which means we should return bus error.
+    if (nread == 0)
+        return PageFaultResponse::BusError;
+
     if (nread < PAGE_SIZE) {
         // If we read less than a page, zero out the rest to avoid leaking uninitialized data.
         memset(page_buffer + nread, 0, PAGE_SIZE - nread);