Btrfs: don't flush the cache inode before writing it

I noticed we had a little bit of latency when writing out the space cache
inodes.  It's because we flush it before we write anything in case we have dirty
pages already there.  This doesn't matter though since we're just going to
overwrite the space, and there really shouldn't be any dirty pages anyway.  This
makes some of my tests run a little bit faster.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>