ppp: fix out-of-bounds access in bpf_prog_create()

sock_fprog_kern::len is in units of struct sock_filter, not bytes.

Fixes: 3e859adf ("compat_ioctl: unify copy-in of ppp filters")
Reported-by:  <syzbot+eb853b51b10f1befa0b7@syzkaller.appspotmail.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>