Btrfs fixes

Hi Linus,

I have a few more fixes in my for-linus-4.1 branch:

git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs.git for-linus-4.1

The first commit is a fix from Filipe for a very old extent buffer reuse race that triggered a BUG_ON. It hasn't come up often, I looked through old logs at FB and we hit it a handful of times over the last year.

The rest are other corners he hit during testing

Filipe Manana (4) commits (+68/-10):
Btrfs: fix race when reusing stale extent buffers that leads to BUG_ON (+19/-0)
Btrfs: fix race between block group creation and their cache writeout (+27/-4)
Btrfs: fix panic when starting bg cache writeout after IO error (+10/-4)
Btrfs: fix crash after inode cache writeback failure (+12/-2)

Total: (4) commits

 fs/btrfs/extent-tree.c      | 31 +++++++++++++++++++++++++++----
 fs/btrfs/extent_io.c        | 19 +++++++++++++++++++
 fs/btrfs/free-space-cache.c | 14 ++++++++++++--
 fs/btrfs/ordered-data.c     | 14 ++++++++++----
 4 files changed, 68 insertions(+), 10 deletions(-)