Revert "NFS: add nfs_sb_deactive_async to avoid deadlock"

This reverts commit 324d003b.

The deadlock turned out to be caused by a workqueue limitation that has
now been worked around in the RPC code (see comment in rpc_free_task).

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>