btrfs: avoid unnecessary root node COW during snapshotting

btrfs: avoid unnecessary root node COW during snapshotting

There's no need to COW the root node of the subvolume we are snapshotting
because we then call btrfs_copy_root(), which creates a copy of the root
node and sets its generation to the current transaction. So remove this
redundant COW right before calling btrfs_copy_root(), saving one extent
allocation, memory allocation, copying things, etc, and making the code
less confusing. Also rename the extent buffer variable from "old" to
"root_eb" since that name no longer makes any sense after removing the
unnecessary COW operation.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>