bug#54783: ZRAM default priority wrong

[Stefan Baums]

I have a swap file and zram configured without specifying
priority. The swap file was assigned -2, and zram -3, with the
effect that zram was never used. I had to manually change the zram
priority to something higher that that of the swap file.

The manual wrongly claims that the default priority for zram is
-1. The same paragraph of the manual also says:

   ‘swapon’ accepts values between -1 and 32767, with higher
   values indicating higher priority.

which makes the -2 priority of the swap file look suspect (though
it seems to work).