fix Tools::df for big storage usage values

fix Tools::df for big storage usage values

if the size/avail of a mount is bigger than a certain amount,
json_encode writes the number in scientific format, which was not
matched by our \d+ regex.

This then resulted in 'undef' values for the result hash and
subsequently led to errors and warnings.

Extend the regex to also match scientific formatted numbers,
perl can then use them as is, no need for any conversion.

https://forum.proxmox.com/threads/bug-in-pve-tools-df-when-adding-petabyte-scale-storage.60090/#post-277050

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>