IB: Put rlimit accounting struct in struct ib_umem

When memory pinned with ib_umem_get() is released, ib_umem_release()
needs to subtract the amount of memory being unpinned from
mm->locked_vm.  However, ib_umem_release() may be called with
mm->mmap_sem already held for writing if the memory is being released
as part of an munmap() call, so it is sometimes necessary to defer
this accounting into a workqueue.

However, the work struct used to defer this accounting is dynamically
allocated before it is queued, so there is the possibility of failing
that allocation.  If the allocation fails, then ib_umem_release has no
choice except to bail out and leave the process with a permanently
elevated locked_vm.

Fix this by allocating the structure to defer accounting as part of
the original struct ib_umem, so there's no possibility of failing a
later allocation if creating the struct ib_umem and pinning memory
succeeds.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

include/rdma/ib_umem.h

@@ -45,6 +45,9 @@ struct ib_umem {
 	int			page_size;
 	int                     writable;
 	struct list_head	chunk_list;
+	struct work_struct	work;
+	struct mm_struct       *mm;
+	unsigned long		diff;
 };
 
 struct ib_umem_chunk {