xfs: account for null transactions in bunmapi

In e1a4e37c ("xfs: try to avoid blowing out the transaction
reservation when bunmaping a shared extent"), we try to constrain the
amount of real extents we unmap from the data fork in a given call so
that we don't blow out transaction reservations.

However, not all bunmapi operations require a transaction -- if we're
only removing a delalloc extent, no transaction is needed, so we have to
code against that.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>