NFSD: Limit the number of concurrent async COPY operations

In the Linux kernel, the following vulnerability has been resolved:

NFSD: Limit the number of concurrent async COPY operations

Nothing appears to limit the number of concurrent async COPY

operations that clients can start. In addition, AFAICT each async

COPY can copy an unlimited number of 4MB chunks, so can run for a

long time. Thus IMO async COPY can become a DoS vector.

Add a restriction mechanism that bounds the number of concurrent

background COPY operations. Start simple and try to be fair -- this

patch implements a per-namespace limit.

An async COPY request that occurs while this limit is exceeded gets

NFS4ERR_DELAY. The requesting client can choose to send the request

again after a delay or fall back to a traditional read/write style

copy.

If there is need to make the mechanism more sophisticated, we can

visit that in future patches.