Kernel/SMP: Don't panic in inode fault if other CPU already paged it in

It may happen that CPU A manages to page in from the same inode
while we're just entering the same page fault handler on CPU B.

Handle it gracefully by checking if the data has already been paged in
(instead of VERIFY'ing that it hasn't) and then remap the page if that's
the case.

Kernel/Memory/Region.cpp

@@ -401,7 +401,16 @@ PageFaultResponse Region::handle_inode_fault(size_t page_index_in_region)
 
     auto page_index_in_vmobject = translate_to_vmobject_page(page_index_in_region);
     auto& vmobject_physical_page_entry = inode_vmobject.physical_pages()[page_index_in_vmobject];
-    VERIFY(vmobject_physical_page_entry.is_null());
+
+    {
+        ScopedSpinLock locker(inode_vmobject.m_lock);
+        if (!vmobject_physical_page_entry.is_null()) {
+            dbgln_if(PAGE_FAULT_DEBUG, "handle_inode_fault: Page faulted in by someone else before reading, remapping.");
+            if (!remap_vmobject_page(page_index_in_vmobject))
+                return PageFaultResponse::OutOfMemory;
+            return PageFaultResponse::Continue;
+        }
+    }
 
     dbgln_if(PAGE_FAULT_DEBUG, "Inode fault in {} page index: {}", name(), page_index_in_region);