…odifying extent tree"

Commit 7a12d84 ("btrfs-progs: Do metadata preallocation as long as
we're not modifying extent tree") tries to fix kdave#123, however due to the
fact that chunk tree also has root->ref_cows set, we will call
do_chunk_alloc() until call stack explodes.

So revert that offending patch until we have a much better comment on
root->ref_cows and find a better solution to this problem.

Signed-off-by: Qu Wenruo <wqu@suse.com>