comedi: get rid of compat_alloc_user_space() mess in COMEDI_INSN compat

Just take copy_from_user() out of do_insn_ioctl() into the caller and
have compat_insn() build a native version and pass it to do_insn_ioctl()
directly.

One difference from the previous commits is that the helper used to
convert 32bit variant to native has two users - compat_insn() and
compat_insnlist().  The latter will be converted in next commit;
for now we simply split the helper in two variants - "userland 32bit
to kernel native" and "userland 32bit to userland native".  The latter
is renamed old get_compat_insn(); it will be gone in the next commit.

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