nfsd: getattr for FATTR4_WORD0_FILES_AVAIL needs the statfs buffer

commit 12337901d654415d9f764b5f5ba50052e9700f37 upstream.

Note nobody's ever noticed because the typical client probably never
requests FILES_AVAIL without also requesting something else on the list.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>