x86: make sure initmem is writable on 64-bit

Impact: unification

This patch ports commit 3c1df68b848b39270752ff8d4b956cc4a4dce0f6 ("x86: make
sure initmem is writable") to the 64-bit version to unify implementations of
free_init_pages().

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Arjan van de Ven <arjan@linux.intel.com>
LKML-Reference: <1236078904.2675.17.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>