compat_ioctl: ppp: move simple commands into ppp_generic.c

All ppp commands that are not already handled in ppp_compat_ioctl()
are compatible, so they can now handled by calling the native
ppp_ioctl() directly.

Without CONFIG_BLOCK, the generic compat_ioctl table is now empty,
so add a check to avoid a build failure in the looking function for
that configuration.

Cc: netdev@vger.kernel.org
Cc: linux-ppp@vger.kernel.org
Cc: Paul Mackerras <paulus@samba.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>