[S390] free_initmem: reduce code duplication

free_initmem() and free_initrd_mem() are nearly identical. So make them
call a common function.
Also fixes a bug: if the initrd wouldn't start on a page boundary also
memory after the initrd would be initialized with the poison value.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>