NFSD: Return nfserr_serverfault if splice_ok but buf->pages have data

[ Upstream commit 06981d560606ac48d61e5f4fff6738b925c93173 ]

This was discussed with Chuck as part of this patch set. Returning
nfserr_resource was decided to not be the best error message here, and
he suggested changing to nfserr_serverfault instead.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Link: https://lore.kernel.org/linux-nfs/20220907195259.926736-1-anna@kernel.org/T/#t


Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>