ipv4: Constrain UFO fragment sizes to multiples of 8 bytes

ipv4: Constrain UFO fragment sizes to multiples of 8 bytes

[ Upstream commit d9be4f7a6f5a8da3133b832eca41c3591420b1ca ]

Because the ip fragment offset field counts 8-byte chunks, ip
fragments other than the last must contain a multiple of 8 bytes of
payload.  ip_ufo_append_data wasn't respecting this constraint and,
depending on the MTU and ip option sizes, could create malformed
non-final fragments.

Google-Bug-Id: 5009328
Signed-off-by: Bill Sommerfeld <wsommerfeld@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>