nfsd: fix NULL dereference in nfsd_statfs()

The commit ebabe9a9


    pass a struct path to vfs_statfs
introduced the struct path initialization, and this seems to trigger
an Oops on my machine.

fh_dentry field may be NULL and set later in fh_verify(), thus the
initialization of path must be after fh_verify().

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>