btrfs: simplify cow only root list extraction during transaction commit

There's no need to keep a local variable to extract the first member of
the list and then do a list_entry() call, we can use list_first_entry()
instead, removing the need for the temporary variable and extracting the
first element in a single step.

Also, there's no need to do a list_del_init() followed by list_add_tail(),
instead we can use list_move_tail(). We are in transaction commit critical
section where we don't need to worry about concurrency and that's why we
don't take any locks and can use list_move_tail() (we do assert early at
commit_cowonly_roots() that we are in the critical section, that the
transaction's state is TRANS_STATE_COMMIT_DOING).

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