fix(resize_btrfs): Fix my bogus shell script.

- mapfile cannot be used in a pipe since a pipeline runs in a subshell.
  Use bash command substitution instead to feed mapfile data.
- Fix obviously bogus awk command for parsing btrfs output.
Merge pull request #60 from marineam/fix-btrfs

fix(resize_btrfs): Fix my bogus shell script.