Make btrfs-progs compile with -fstrict-aliasing (included in -O2) again.

Make btrfs-progs compile with -fstrict-aliasing (included in -O2) again.

This is done by doing a two-step conversion (rather than a one-step).
First, the variable goes from type * to void *, and then to
implicitly to void **.
(Not sure if this is "good practice", but it shuts up the compiler,
so it seems the compiler takes into account that we are actually punning
it this way.)