new helper: uaccess_kernel()

commit db68ce10c4f0a27c1ff9fa0e789e5c41f8c4ea63 upstream.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[only take the include/linux/uaccess.h portion - gregkh]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>