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.