For defrag, we don't really want to use btrfs_get_extent() to iterate
all extent maps of an inode.

The reasons are:

- btrfs_get_extent() can merge extent maps
  And the result em has the higher generation of the two, causing defrag
  to mark unnecessary part of such merged large extent map.

  This in fact can result extra IO for autodefrag in v5.16+ kernels.

  However this patch is not going to completely solve the problem, as
  one can still using read() to trigger extent map reading, and got
  them merged.

  The completely solution for the extent map merging generation problem
  will come as an standalone fix.

- btrfs_get_extent() caches the extent map result
  Normally it's fine, but for defrag the target range may not get
  another read/write for a long long time.
  Such cache would only increase the memory usage.

- btrfs_get_extent() doesn't skip older extent map
  Unlike the old find_new_extent() which uses btrfs_search_forward() to
  skip the older subtree, thus it will pick up unnecessary extent maps.

This patch will fix the regression by introducing defrag_get_extent() to
replace the btrfs_get_extent() call.

This helper will:

- Not cache the file extent we found
  It will search the file extent and manually convert it to em.

- Use btrfs_search_forward() to skip entire ranges which is modified in
  the past

This should reduce the IO for autodefrag.
Reported-by: Filipe Manana <fdmanana@suse.com>
Fixes: 7b508037 ("btrfs: defrag: use defrag_one_cluster() to implement btrfs_defrag_file()")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>