helpers: use float for format_filesize()

helpers: use float for format_filesize()

In format_filesize(), we really do want float division,
in order to display the value correctly. For example, if
there's a file with 45200000 bytes, that should be shown
as 43.1 MB, not 43.0. Fix this by using proper float
division here, not int division.

Fixes: a5809723352c ("helpers: use // not / for division")
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>