staging/lustre: Add __user attributes to libcfs_ioctl_get/popdata args

Just make clear which pointers are from userspace and which are not in
libcfs_ioctl_getdata/libcfs_ioctl_popdata and their callers.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>