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