btrfs-progs: [trivial] licence correction

[tobias . geerinckx . rice]

Hullo,

It was pointed out to me that the current licence (‘gpl2+’) for
btrfs-progs is just wrong and should be ‘gpl2’ instead.

I must have opened one of the few gpl2+-licenced files in the tarball
by sheer bad luck. Here is a patch to set things right.

Kind regards,

T G-R

(I struggled to write a ‘GNU-style’ commit message for something so
trivial and probably went overboard. This can be fixed in a later
patch series.)