mm: move dummy_vm_ops out of a header

Otherwise the kernel ends up with multiple copies:
$ nm vmlinux | grep dummy_vm_ops
ffffffff81e4ea00 d dummy_vm_ops.2
ffffffff81e11760 d dummy_vm_ops.254
ffffffff81e406e0 d dummy_vm_ops.4
ffffffff81e3c780 d dummy_vm_ops.7

While here prefix it with vma_.

Link: https://lkml.kernel.org/r/20230806231611.1395735-1-mjguzik@gmail.com
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/init-mm.c

@@ -17,6 +17,8 @@
 #define INIT_MM_CONTEXT(name)
 #endif
 
+const struct vm_operations_struct vma_dummy_vm_ops;
+
 /*
  * For dynamically allocated mm_structs, there is a dynamically sized cpumask
  * at the end of the structure, the size of which depends on the maximum CPU