NFSD: Add experimental setting to disable the use of splice read

NFSD currently has two separate code paths for handling read
requests. One uses page splicing; the other is a traditional read
based on an iov iterator.

Because most Linux file systems support splice read, the latter
does not get nearly the same test experience as splice reads.

To force the use of vectored reads for testing and benchmarking,
introduce the ability to disable splice reads for all NFS READ
operations.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>