fix(resize_btrfs): Search by device name instead of mount point.

It seems the `rootfs / rootfs rw 0 0` entry in the mount table is
confusing to `btrfs fi show`, causing it to exit with code 1 despite
there actually being a btrfs filesystem mounted to / later in the table.

Or at least that's what my initial reading of the code implies. In
either case using -d option to prevent it from scanning mount points
works around the bug.

Fixes https://github.com/coreos/bugs/issues/3