Return the correct error for Seek at EOF.
The Linux NFSv4.2 client expects an NFSERR_NXIO error to be returned
when a NFS4_CONTENT_DATA request with sa_offset == file_size is done for Seek.
(This is not actually what RFC-7862 says is the correct reply for this
operation request, so it is only done when vfs.nfsd.linux42server is set.)
Anyhow, without this patch the code erroeously returned NFSERR_INVAL.