Introduce a new helper, defrag_collect_targets(), to collect all
possible targets to be defragged.

This function will not consider things like max_sectors_to_defrag, thus
caller should be responsible to ensure we don't exceed the limit.

This function will be the first stage of later defrag rework.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/ioctl.c | 120 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 120 insertions(+)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index c0639780f99c..043c44daa5ae 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c

@@ -1427,6 +1427,126 @@ static int cluster_pages_for_defrag(struct inode *inode,
 
 }
 
+struct defrag_target_range {
+	struct list_head list;
+	u64 start;
+	u64 len;
+};
+
+/*
+ * Helper to collect all valid target extents.