BTRFS: support NFSv2 export

The "fh_len" passed to ->fh_to_* is not guaranteed to be that same as
that returned by encode_fh - it may be larger.

With NFSv2, the filehandle is fixed length, so it may appear longer
than expected and be zero-padded.

So we must test that fh_len is at least some value, not exactly equal
to it.

Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: David Sterba <dsterba@suse.cz>

fs/btrfs/export.c

@@ -112,11 +112,11 @@ static struct dentry *btrfs_fh_to_parent(struct super_block *sb, struct fid *fh,
 	u32 generation;
 
 	if (fh_type == FILEID_BTRFS_WITH_PARENT) {
-		if (fh_len !=  BTRFS_FID_SIZE_CONNECTABLE)
+		if (fh_len <  BTRFS_FID_SIZE_CONNECTABLE)
 			return NULL;
 		root_objectid = fid->root_objectid;
 	} else if (fh_type == FILEID_BTRFS_WITH_PARENT_ROOT) {
-		if (fh_len != BTRFS_FID_SIZE_CONNECTABLE_ROOT)
+		if (fh_len < BTRFS_FID_SIZE_CONNECTABLE_ROOT)
 			return NULL;
 		root_objectid = fid->parent_root_objectid;
 	} else
@@ -136,11 +136,11 @@ static struct dentry *btrfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
 	u32 generation;
 
 	if ((fh_type != FILEID_BTRFS_WITH_PARENT ||
-	     fh_len != BTRFS_FID_SIZE_CONNECTABLE) &&
+	     fh_len < BTRFS_FID_SIZE_CONNECTABLE) &&
 	    (fh_type != FILEID_BTRFS_WITH_PARENT_ROOT ||
-	     fh_len != BTRFS_FID_SIZE_CONNECTABLE_ROOT) &&
+	     fh_len < BTRFS_FID_SIZE_CONNECTABLE_ROOT) &&
 	    (fh_type != FILEID_BTRFS_WITHOUT_PARENT ||
-	     fh_len != BTRFS_FID_SIZE_NON_CONNECTABLE))
+	     fh_len < BTRFS_FID_SIZE_NON_CONNECTABLE))
 		return NULL;
 
 	objectid = fid->objectid;