nfsd: getattr for FATTR4_WORD0_FILES_AVAIL needs the statfs buffer

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>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>