compat_ioctl: move PPPIOCSCOMPRESS to ppp_generic

Rather than using a compat_alloc_user_space() buffer, moving
this next to the native handler allows sharing most of
the code, leaving only the user copy portion distinct.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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>