btrfs: add missing RCU unlock in error path in try_release_subpage_extent_buffer()

In the Linux kernel, the following vulnerability has been resolved: btrfs: add missing RCU unlock in error path in try_release_subpage_extent_buffer() Call rcu_read_lock() before exiting the loop in try_release_subpage_extent_buffer() because there is a rcu_read_unlock() call past the loop. This has been detected by the Clang thread-safety analyzer.