xfs: reorder the minlen and prod calculations in xfs_bmap_rtalloc

xfs_bmap_rtalloc is a bit of a mess in terms of calculating the locally
need variables.  Reorder them a bit so that related code is located
next to each other - the raminlen calculation moves up next to where
the maximum len is calculated, and all the prod calculation is move
into a single place and rearranged so that the real prod calculation
only happens when it actually is needed.

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