xfs: block deltas in xfs_trans_unreserve_and_mod_sb must be positive

And to make that more clear, rearrange the code a bit and add asserts
and a comment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>