kernel: move use_mm/unuse_mm to kthread.c

cover the newly merged use_mm/unuse_mm caller in vfio

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Link: http://lkml.kernel.org/r/20200416053158.586887-2-hch@lst.de
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/vfio/vfio_iommu_type1.c

@@ -27,7 +27,7 @@
 #include <linux/iommu.h>
 #include <linux/module.h>
 #include <linux/mm.h>
-#include <linux/mmu_context.h>
+#include <linux/kthread.h>
 #include <linux/rbtree.h>
 #include <linux/sched/signal.h>
 #include <linux/sched/mm.h>