Kernel: Make Region creation API OOM safe

- Make Region::create_kernel_only OOM safe.

- Make Region::create_user_accessible mostly OOM safe, there are still
  some tendrils to untangle before it and be completely fixed.

Kernel/VM/Region.cpp

@@ -210,15 +210,16 @@ size_t Region::amount_shared() const
 
 NonnullOwnPtr<Region> Region::create_user_accessible(Process* owner, const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, OwnPtr<KString> name, Region::Access access, Cacheable cacheable, bool shared)
 {
-    auto region = adopt_own(*new Region(range, move(vmobject), offset_in_vmobject, move(name), access, cacheable, shared));
-    if (owner)
+    auto region = adopt_own_if_nonnull(new Region(range, move(vmobject), offset_in_vmobject, move(name), access, cacheable, shared));
+    if (region && owner)
         region->m_owner = owner->make_weak_ptr();
-    return region;
+    // FIXME: Return OwnPtr and propagate failure, currently there are too many assumptions made by down stream callers.
+    return region.release_nonnull();
 }
 
-NonnullOwnPtr<Region> Region::create_kernel_only(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, OwnPtr<KString> name, Region::Access access, Cacheable cacheable)
+OwnPtr<Region> Region::create_kernel_only(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, OwnPtr<KString> name, Region::Access access, Cacheable cacheable)
 {
-    return adopt_own(*new Region(range, move(vmobject), offset_in_vmobject, move(name), access, cacheable, false));
+    return adopt_own_if_nonnull(new Region(range, move(vmobject), offset_in_vmobject, move(name), access, cacheable, false));
 }
 
 bool Region::should_cow(size_t page_index) const

Kernel/VM/Region.h

@@ -51,7 +51,7 @@ public:
     };
 
     static NonnullOwnPtr<Region> create_user_accessible(Process*, const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, OwnPtr<KString> name, Region::Access access, Cacheable, bool shared);
-    static NonnullOwnPtr<Region> create_kernel_only(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, OwnPtr<KString> name, Region::Access access, Cacheable = Cacheable::Yes);
+    static OwnPtr<Region> create_kernel_only(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, OwnPtr<KString> name, Region::Access access, Cacheable = Cacheable::Yes);
 
     ~Region();