xfs: factor out a xfs_rtalloc_sumlevel helper

xfs_rtallocate_extent_size has two loops with nearly identical logic
in them.  Split that logic into a separate xfs_rtalloc_sumlevel helper.

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>