xfs: return the allocated transaction from xfs_trans_alloc_empty

xfs: return the allocated transaction from xfs_trans_alloc_empty

xfs_trans_alloc_empty can't return errors, so return the allocated
transaction directly instead of an output double pointer argument.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>