staging: erofs: set sb->s_root to NULL when failing from __getname()

Set sb->s_root to NULL when failing from __getname(),
so that we can avoid double dput and unnecessary operations
in generic_shutdown_super().

Signed-off-by: Chengguang Xu <cgxu519@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/erofs/super.c

@@ -457,6 +457,7 @@ static int erofs_read_super(struct super_block *sb,
 	 */
 err_devname:
 	dput(sb->s_root);
+	sb->s_root = NULL;
 err_iget:
 #ifdef EROFS_FS_HAS_MANAGED_CACHE
 	iput(sbi->managed_cache);