NFSD: Fix bad checking of space for padding in splice read

Note that the caller has already reserved space for count and eof, so
xdr->p has already moved past them, only the padding remains.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Fixes dc97618d (nfsd4: separate splice and readv cases)
Signed-off-by: J. Bruce Fields <bfields@redhat.com>