exofs: oops after late failure in mount

We have already set ->s_root, so ->put_super() is going to be called.
Freeing ->s_fs_info is a bloody bad idea when it's going to be
dereferenced very shortly...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/exofs/super.c

@@ -838,6 +838,8 @@ static int exofs_fill_super(struct super_block *sb, void *data, int silent)
 	ret = bdi_setup_and_register(&sbi->bdi, "exofs", BDI_CAP_MAP_COPY);
 	if (ret) {
 		EXOFS_DBGMSG("Failed to bdi_setup_and_register\n");
+		dput(sb->s_root);
+		sb->s_root = NULL;
 		goto free_sbi;
 	}