NFS: Don't allow waiting for exiting tasks

[ Upstream commit 8d3ca331 ]

Once a task calls exit_signals() it can no longer be signalled. So do
not allow it to do killable waits.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>