net/drivers/ieee1275/ofnet: Fix incorrect netmask

net/drivers/ieee1275/ofnet: Fix incorrect netmask

The netmask configured in firmware is not respected on ppc64 (big endian).
When 255.255.252.0 is set as netmask in firmware, the following is the
value of bootpath string in grub_ieee1275_parse_bootpath():

  /vdevice/l-lan@30000002:speed=auto,duplex=auto,192.168.88.10,,192.168.89.113,192.168.88.1,5,5,255.255.252.0,512

The netmask in this bootpath is not a problem, since it's a value specified
in firmware. But the value of subnet_mask.ipv4 was set with 0xfffffc00, and
__builtin_ctz(~grub_le_to_cpu32(subnet_mask.ipv4)) returned 16 (not 22).
As a result, 16 was used for netmask wrongly:

  1111 1111 1111 1111 1111 1100 0000 0000 # subnet_mask.ipv4(=0xfffffc00)
  0000 0000 1111 1100 1111 1111 1111 1111 # grub_le_to_cpu32(subnet_mask.ipv4)
  1111 1111 0000 0011 0000 0000 0000 0000 # ~grub_le_to_cpu32(subnet_mask.ipv4)

and the count of zero with __builtin_ctz() can be 16. This patch changes
it as below:

  1111 1111 1111 1111 1111 1100 0000 0000 # subnet_mask.ipv4(=0xfffffc00)
  0000 0000 1111 1100 1111 1111 1111 1111 # grub_le_to_cpu32(subnet_mask.ipv4)
  1111 1111 1111 1111 1111 1100 0000 0000 # grub_be_to_cpu32(subnet_mask.ipv4)
  0000 0000 0000 0000 0000 0011 1111 1111 # ~grub_be_to_cpu32(subnet_mask.ipv4)

The count of zero with __builtin_clz() can be 22 (clz counts the number
of one bits preceding the most significant zero bit).

Signed-off-by: Masahiro Matsuya <mmatsuya@redhat.com>
Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>