btrfs: avoid starting new transaction and commit in relocate_block_group()

btrfs: avoid starting new transaction and commit in relocate_block_group()

We join a transaction with the goal of catching the current transaction
and then commit it to get rid of pinned extents and reclaim free space,
but a join can create a new transaction if there isn't any running, and if
right before we did the join the current transaction happened to be
committed by someone else (like the transaction kthread for example),
we end up starting and committing a new transaction, causing rotation of
the super block backup roots besides extra and useless IO.

So instead of doing a transaction join followed by a commit, use the
helper btrfs_commit_current_transaction() which ensures no transaction is
created if there isn't any running.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>