Kernel: Enforce W^X between sys$mmap() and sys$execve()

It's now an error to sys$mmap() a file as writable if it's currently
mapped executable by anyone else.

It's also an error to sys$execve() a file that's currently mapped
writable by anyone else.

This fixes a race condition vulnerability where one program could make
modifications to an executable while another process was in the kernel,
in the middle of exec'ing the same executable.

Test: Kernel/elf-execve-mmap-race.cpp

Kernel/VM/InodeVMObject.h

@@ -47,6 +47,9 @@ public:
 
     int release_all_clean_pages();
 
+    u32 writable_mappings() const;
+    u32 executable_mappings() const;
+
 private:
     explicit InodeVMObject(Inode&, size_t);
     explicit InodeVMObject(const InodeVMObject&);