[RFC] log mode COW

["Yan, Zheng" <zheng.yan@oracle.com>]

The aim of log mode COW is solve extent tree performance issue when
free metadata space are scarce and fragmented. The basic idea is:
when cowing a non-shared block, we insert a log entry that maps
the new block to the old block into a special log tree, and do not
update the extent tree. After transaction is fully committed, copy
the new block back to the old location.

Log mode COW can be enabled/disabled by log_mode=%d mount option.
'0' means disable log mode COW; '1' means enable log for tree blocks
that are not reference counted; '2' means enable log for all tree
blocks are not shared.

NOTE: The patch series base on current btrfs-unstable. They are for
      test and review only. Enable log mode COW changes disk format.

Regards
Yan, Zheng