Simplify calculation of vol size when adding a btrfs subvol. (#1033356)
Simplify calculation of vol size when adding a btrfs subvol. (#1033356)

The previous method of summing the subvolumes' req_size doesn't work
when the initial layout is a product of autopart since the base sizes
do not match the actual sizes due to growth.

rvykydal/blivet,rhinstaller/blivet,vojtechtrefny/blivet,rhinstaller/blivet,rvykydal/blivet,jkonecny12/blivet,vojtechtrefny/blivet,AdamWill/blivet,dwlehman/blivet,AdamWill/blivet,dwlehman/blivet,vpodzime/blivet,jkonecny12/blivet,vpodzime/blivet