NFSD: Use vfs_iocb_iter_read()

NFSD: Use vfs_iocb_iter_read()

Refactor: Enable the use of IOCB flags to control NFSD's individual
read operations (when not using splice). This allows the eventual
use of atomic, uncached, direct, or asynchronous reads.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>