NFS: unmark NFS direct I/O as experimental

Remove the EXPERIMENTAL flag from the NFS_DIRECTIO option.

Test plan:
Unset the EXPERIMENTAL kernel build option and check to see that the NFS
direct I/O option is still available.

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