Slightly improve currentSize for btrfs volumes.
Slightly improve currentSize for btrfs volumes.

It's better than the size of the first member device. This was leading to
a max size smaller than the current size for multi-device btrfs
filesystems.

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