nfs: declare nfs_xdev_mount as static

Sparse warning:
fs/nfs/super.c:2517:15: warning: symbol 'nfs_xdev_mount' was not declared.
Should it be static?

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>