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>

fs/nfsd/vfs.c

@@ -2033,15 +2033,17 @@ nfsd_readdir(struct svc_rqst *rqstp, struct svc_fh *fhp, loff_t *offsetp,
 __be32
 nfsd_statfs(struct svc_rqst *rqstp, struct svc_fh *fhp, struct kstatfs *stat, int access)
 {
-	struct path path = {
-		.mnt	= fhp->fh_export->ex_path.mnt,
-		.dentry	= fhp->fh_dentry,
-	};
 	__be32 err;
 
 	err = fh_verify(rqstp, fhp, 0, NFSD_MAY_NOP | access);
-	if (!err && vfs_statfs(&path, stat))
-		err = nfserr_io;
+	if (!err) {
+		struct path path = {
+			.mnt	= fhp->fh_export->ex_path.mnt,
+			.dentry	= fhp->fh_dentry,
+		};
+		if (vfs_statfs(&path, stat))
+			err = nfserr_io;
+	}
 	return err;
 }