btrfs: fix race in btrfs_free_dummy_fs_info()

We drop the lock which protects the radix tree, so we must call
radix_tree_iter_next() in order to avoid a modification to the tree
invalidating the iterator state.

Link: http://lkml.kernel.org/r/1480369871-5271-54-git-send-email-mawilcox@linuxonhyperv.comSigned-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Tested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/btrfs/tests/btrfs-tests.c

@@ -162,6 +162,7 @@ void btrfs_free_dummy_fs_info(struct btrfs_fs_info *fs_info)
 				slot = radix_tree_iter_retry(&iter);
 			continue;
 		}
+		slot = radix_tree_iter_next(&iter);
 		spin_unlock(&fs_info->buffer_lock);
 		free_extent_buffer_stale(eb);
 		spin_lock(&fs_info->buffer_lock);