cxl: Only check pid for userspace contexts

We only need to check the pid attached to this context for userspace contexts.
Kernel contexts can skip this check.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>