This is part of SVM profiling API, export sysfs counters for
per-process, per-GPU vm retry fault, pages migrated in and out of GPU vram.

counters will not be updated in parallel in GPU retry fault handler and
migration to vram/ram path, use READ_ONCE to avoid compiler
optimization.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>