Kernel/Memory: Add option to annotate region mapping as immutable

We add this basic functionality to the Kernel so Userspace can request a
particular virtual memory mapping to be immutable. This will be useful
later on in the DynamicLoader code.

The annotation of a particular Kernel Region as immutable implies that
the following restrictions apply, so these features are prohibited:
- Changing the region's protection bits
- Unmapping the region
- Annotating the region with other virtual memory flags
- Applying further memory advises on the region
- Changing the region name
- Re-mapping the region

Kernel/Memory/AddressSpace.cpp

@@ -59,6 +59,8 @@ ErrorOr<void> AddressSpace::unmap_mmap_range(VirtualAddress addr, size_t size)
     if (auto* whole_region = find_region_from_range(range_to_unmap)) {
         if (!whole_region->is_mmap())
             return EPERM;
+        if (whole_region->is_immutable())
+            return EPERM;
 
         PerformanceManager::add_unmap_perf_event(Process::current(), whole_region->range());
 
@@ -69,6 +71,8 @@ ErrorOr<void> AddressSpace::unmap_mmap_range(VirtualAddress addr, size_t size)
     if (auto* old_region = find_region_containing(range_to_unmap)) {
         if (!old_region->is_mmap())
             return EPERM;
+        if (old_region->is_immutable())
+            return EPERM;
 
         // Remove the old region from our regions tree, since were going to add another region
         // with the exact same start address.
@@ -99,6 +103,8 @@ ErrorOr<void> AddressSpace::unmap_mmap_range(VirtualAddress addr, size_t size)
     for (auto* region : regions) {
         if (!region->is_mmap())
             return EPERM;
+        if (region->is_immutable())
+            return EPERM;
     }
 
     Vector<Region*, 2> new_regions;