3.11/btrfs lz4

Add LZ4 compression used in kernel into btrfs. Based on http://repo.or.cz/w/linux-2.6/btrfs-unstable.git/shortlog/refs/heads/dev/lz4-311
Tested by me since 3.11-rc1.

WARNING! Incompatible with 3.9/btrfs-lz4: in my case there are decompression errors in some file  on old partitions.