PaX/GRSecurity compatibility: Mark zio_data_buf_* with KM_USER

As of Linux 3.14.y, the PaX/GRSecurity patch set will panic the kernel
if copy_to_user()/copy_from_user() occurs on a memory allocation that is
not marked with GFP_USERCOPY when CONFIG_PAX_USERCOPY_SLABS is set on
the kernel. We resolve this by marking `zio_data_buf_*` with a KM_USER
flag that the SPL will translate into GFP_USERCOPY.

Signed-off-by: Richard Yao <ryao@gentoo.org>