luci-app-vnstat2: use more efficient way to get database interfaces

The currently used command "vnstat --json f 1" can be very slow when the
database contains many entries. This can result in an XHR timeout error
on the configuration page.

Use the "--dbiflist" option instead, which was added in vnStat 2.7.

Fixes: #6702
Signed-off-by: Jan Hoffmann <jan@3e8.eu>