NFSD: Never decrement pending_async_copies on error

The error flow in nfsd4_copy() calls cleanup_async_copy(), which
already decrements nn->pending_async_copies.

Reported-by: Olga Kornievskaia <okorniev@redhat.com>
Fixes: aadc3bbe ("NFSD: Limit the number of concurrent async COPY operations")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>