xfs: decouple xfs_trans_alloc_empty from xfs_trans_alloc

xfs_trans_alloc_empty only shares the very basic transaction structure
allocation and initialization with xfs_trans_alloc.

Split out a new __xfs_trans_alloc helper for that and otherwise decouple
xfs_trans_alloc_empty from xfs_trans_alloc.

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