ip*_mc_gsfget(): lift copyout of struct group_filter into callers

pass the userland pointer to the array in its tail, so that part
gets copied out by our functions; copyout of everything else is
done in the callers.  Rationale: reuse for compat; the array
is the same in native and compat, the layout of parts before it
is different for compat.

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