btrfs: take commit root semaphore when iterating in mark_block_group_to_copy()

In the Linux kernel, the following vulnerability has been resolved: btrfs: take commit root semaphore when iterating in mark_block_group_to_copy() mark_block_group_to_copy() iterates over the commit root with skip_locking=true. A concurrent transaction commit can swap and free the commit root during iteration, causing use-after-free when accessing extent buffers. Fix it by using path->need_commit_sem to protect the commit root search.

Where it's fixed