module: merge module_alloc() finally

As Christoph Hellwig suggested, module_alloc() actually can be
unified for i386 and x86_64 (of course, also UML).

Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: 'Ingo Molnar' <mingo@elte.hu>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>