vfs: add "device" tag to /proc/self/mountstats

nfsiostat was failing to find mounted filesystems on kernels after
2.6.38 because of changes to show_vfsstat() by commit
c7f404b4

.  This patch adds back the
"device" tag before the nfs server entry so scripts can parse the
mountstats file correctly.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
CC: stable@kernel.org [>=2.6.39]
Signed-off-by: Christoph Hellwig <hch@lst.de>