IB/umem: Use the correct mm during ib_umem_release

User-space may invoke ibv_reg_mr and ibv_dereg_mr in different threads.

If ibv_dereg_mr is called after the thread which invoked ibv_reg_mr has
exited, get_pid_task will return NULL and ib_umem_release will not
decrease mm->pinned_vm.

Instead of using threads to locate the mm, use the overall tgid from the
ib_ucontext struct instead. This matches the behavior of ODP and
disassociate in handling the mm of the process that called ibv_reg_mr.

Cc: <stable@vger.kernel.org>
Fixes: 87773dd56d ("IB: ib_umem_release() should decrement mm->pinned_vm from ib_umem_get")
Signed-off-by: Lidong Chen <lidongchen@tencent.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

include/rdma/ib_umem.h

@@ -48,7 +48,6 @@ struct ib_umem {
 	int                     writable;
 	int                     hugetlb;
 	struct work_struct	work;
-	struct pid             *pid;
 	struct mm_struct       *mm;
 	unsigned long		diff;
 	struct ib_umem_odp     *odp_data;