powerpc/kexec: Fix loading of kernel + initramfs with kexec_file_load()

Commit b6664ba4 ("s390, kexec_file: drop arch_kexec_mem_walk()")
changed kexec_add_buffer() to skip searching for a memory location if
kexec_buf.mem is already set, and use the address that is there.

In powerpc code we reuse a kexec_buf variable for loading both the
kernel and the initramfs by resetting some of the fields between those
uses, but not mem. This causes kexec_add_buffer() to try to load the
kernel at the same address where initramfs will be loaded, which is
naturally rejected:

  # kexec -s -l --initrd initramfs vmlinuz
  kexec_file_load failed: Invalid argument

Setting the mem field before every call to kexec_add_buffer() fixes
this regression.

Fixes: b6664ba4

 ("s390, kexec_file: drop arch_kexec_mem_walk()")
Cc: stable@vger.kernel.org # v5.0+
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Reviewed-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>