1.5. Faster performance and better data recover in Btrfs

This release adds a few changes that should improve Btrfs performance in many different areas. First, Btrfs replaced the locking primitive used in the core b-tree implementation. A homegrown locking scheme had been used, but now it uses the standard Linux read-write semaphores; this results in much better performance for highly concurrent workloads. There are also a few changes that significantly improve the throughput/latency of dbench-like workloads.

Aditionally, this release implements new data recovery mount options for people with corrupted file systems. rescue=ignoredatacsums, which ignores faileddata checksums, rescue=ignorebadroots, which attempts to continue mounting the file system in read-only mode even if there are corrupted tree roots, and for convenience a new rescue=all option has been added, which enables 'ignorebadroots', 'ignoredatacsums' and 'nologreplay' at the same time.