compat_ioctl: unify copy-in of ppp filters

compat_ioctl: unify copy-in of ppp filters

Now that isdn4linux is gone, the is only one implementation of PPPIOCSPASS
and PPPIOCSACTIVE in ppp_generic.c, so this is where the compat_ioctl
support should be implemented.

The two commands are implemented in very similar ways, so introduce
new helpers to allow sharing between the two and between native and
compat mode.

Signed-off-by: Al Viro <[email protected]>
[arnd: rebased, and added changelog text]
Cc: [email protected]
Cc: [email protected]
Cc: Paul Mackerras <[email protected]>
Cc: "David S. Miller" <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>