smb: fix free space calculation

Summary:
- collapse statvfs call, there's no purpose in manually opening the file
  when libsmbc has a convenience wrapper that does exactly what we need
  with less code on our end
- blockSize is now calculated as per the conditional branches in
  smbc_stat.c
- free space calculation is now backwards compatible with slightly busted
  legacy versions of libsmbc not setting f_bavail

FIXED-IN: 19.12.3
BUG: 298801

Test Plan: free space calculation now works with windows10, ubuntu18.04, ubuntu19.10

Reviewers: ngraham, meven

Reviewed By: ngraham, meven

Subscribers: meven, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D27294