x86: move free_initrd_mem() to common mm/init.c

Impact: cleanup

The function is identical on 32-bit and 64-bit configurations so move it to the
common mm/init.c file.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <1236158020.29024.28.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>