nfsd: Record NFSv4 pre/post-op attributes as non-atomic

For the case of NFSv4, specify to the client that the pre/post-op
attributes were not recorded atomically with the main operation.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>